posting this here to get all my thoughts out so i can think of some other stuff. here is my definitive level 1 realgam guide
items
oran is excellent and i always bring it. the muscle band/wise glasses you start with are generally decent but suboptimal in my experience. if your mon has a high attack or special attack rank, shoot for choice band or specs. if you have low attacking stats but high defenses, go for leftovers or attack-boosting berries (kelpsy/hondew). berries are hella cheap so you should be spamming them early game (sitrus/enigma are good), plus consumable spam is generally a lot of fun cause it leads to exciting games where you find opportunities to switch out. if all your stats are low then get lucky egg.
besides that you don't really need much else, eject button/red card are good for regen users and matchup control (which i will elaborate on later) and you can also play around with stuff like shell bell ebelt exp share, these are good in certain situations but obviously you need a way to get into those situations. make a plan, it's your game not mine
general strategy
try to make sure that you have a mon that matches up well into every possible opposing slot. additionally you don't want any of your mons to be overloaded. if you have 1 mon tasked with answering two threats then you will often just lose. your ideal situation is that each of your 3 mons can go equal to better against an opposing mon. when building i like starting with a mon that matches up generally well vs everything, cause then it's very easy to fit in more niche stuff.
for example against shauna
here i started with drapion who wins vs the fairy, is slightly better into the starter, and only really loses to furfrou-diamond, then sawk was added to trade into the furfrou/goodra slot and i could fit non-imposter ditto, a generally ridiculous mon who's slightly worse into most things and terrible into any taunt mon, as the third.
you get second send always which is a pretty sizable advantage in level 1. at team preview you should be assigning each of your mons an appropriate role in the game (who do i want this in vs?), ideally with some flexibility so opposing red card or hax don't completely throw you off.
inevitably you will lose control of the matchup at some point. the most straightforward way for this to happen is you knock out a mon and the next guy comes in to take you out, or the ref has red card. if this is too threatening for your team, bring your own eject button/red card and hard switch to a mon with it so you can take back control. this is resource-intensive though so it's best to just go with the flow and slowly maneuver your way to good control again if you can.
hard switches from the ref are almost never threatening if you have an answer to the mon coming in. just counterswitch and they're back to square one.
sim overview
tierno (talonflame/oricorio, roserade/lilligant-hisui, mr. rime/crawdaunt) is one of the scarier sims primarily due to sleep powder grasses, who are hard to answer at level 1 unless you bring chesto berry, and also have other tricks in the form of hustle lilligant/poison point roserade. i think the best strategy is to spam grass answers so you can never get forced into an awkward position by these two. the dances are generally not scary post-nerf. talonflame/oricorio struggle in most neutral matchups although talon can burn you which is a little annoying. the rime/daunt slot is whatever.
trevor (raichu/florges, clawitzer/dragalge, aerodactyl/aurorus/tyrantrum) has the fossil slot (aero/tyrant/aurorus), so you'd think you can just bring a water or whatever but in practice this doesn't work because they get chewed up by tyrantrum and also lose to the other slots, and having a dead slot in realgams is generally very, very bad. the fossils generally have a fair bit of sauce and can overpower your answers with stuff like weakness policy aurorus. i think it's best to focus on exploiting the first two slots and going even vs the fossil.
shauna (furfrou/goodra, aromatisse/slurpuff, chesnaught/delphox/greninja) has a fairly sauce-less fairy-type that you can trade into if your mon has a poor matchup vs other things, but said other things are also scary. furfrou/goodra is actually extremely strong and will put up a fight even in a type disadvantage. delphox has fire gem flamethrower which hits for a billion and takes your item after, gren is gren. the two main strategies here are 1) have slot 1 and slot 3 covered, then trade your worst mon into the fairy, and 2) overwhelm the fairy with a steel or poison then frantically try not to die to the other two (see
this match).
team star (2 of dark + klawf, fire + bombirdier, poison + orthworm, fairy + brambleghast, fighting + veluza) makes you learn doubles. matchups are mechanically different here so you will have to adjust your play accordingly, but the core principle of having a punish for each opposing mon holds. also important to note that first order can actually be very annoying here unlike in singles, although the ref's gen 9 mons don't have too many tricks. status such as burn and paralysis is extremely strong into team star, who has no clerics and no immunities besides schedar into burn.
leveling up bad mons
some people (myself included) choose to get a little silly with their mon selection. the question of how to make these mons meaningfully participate in the game is an age-old one, but at level 1 not exceptionally hard to answer because mons mostly do the same thing within their type. so i will just list every type and where you should bring the mon.
normal: absolutely take these into shauna. they can take their chances with furfrou/goodra if they have a decent attacking stat, but more relevant is the fact that they actually do very well into gren. the strongest attack with protean is the unconvincing brick break, meaning you can go for an even trade with even mediocre mons such as pre-toxic boost zangoose or lucky egg linoone/bibarel (who will be using thunder wave + tbolt). of course if neither of these pan out you can always just go for the fairy.
fire: even the most humble of fires generally have an attacking stat sufficient for melting tierno's grasses. bring life orb if you want for extra power. mons that risk a bad matchup such as the heart guy into roserade or pyroar into lilligant can still generally get it done. you can also try bringing these guys to team star to wisp everything, just watch out for schedar + rocky payload bombirdier and veluza. veluza is always scary
water: as i discussed earlier don't bring these into trevor with the sole idea of beating the fossil (though if you can fight the other slots they do fine, i'm bringing gorebyss with the idea of beating slot 2 for instance). they do generally well into team star; brambleghast is typically one of the less scary opponents, and you can dish out damage with rain dance into surf even if you're literally lucky egg luvdisc. the 2 things to note are 1) bring damage-boosting if you can so you aren't a passive loser that slowly dies to poison 2) have a teammate who can eat surfs so you aren't reliant on water pulse to deal damage. physical waters will struggle more due to high def on starmobiles/orthworm/etc but liquidation is a good move, you'll be fine
grass: stay away from tierno. if you have sleep powder you can take on shauna (beware team star, starmobiles have overcoat and are immune), if you don't have sleep powder just try to beat trevor's first slot. razor claw leaf blade will hurt florges and raichu struggles to do anything. of course dragalge and some of the fossils just own you so don't expect to get much better than a 1-for-1.
electric: paralysis is broken in level 1. even extremely unserious mons like pachirisu can either outright beat tierno's bird or stay around forever vs team star paralyzing everyone while an aggressive partner simply feasts. thunder shock is there for even more fun. i would say mostly leave shauna and trevor alone as they have clerics to ruin your fun.
bug: these are good into tierno as long as you can withstand fiery dance. you beat slot 3 unless you're bug/flying or bug/fire, in which case you just go for the grasses, or you're bug/rock, in which case you beat the birds.
fighting: it's very very hard to find one of these that isn't at least decent at punching things. these do alright vs the trevor fossil and shauna's slot 1 but generally require more thoughtful building. they win less hard vs things but also lose less hard. they're generally unfavorable vs star but if you need a counter to giacomo in particular they got you covered, and they're good at clicking rock slide.
psychic: if you have twave you can send these into tierno's grass and slap on a choice specs and this will work even if you are grumpig or chimecho. these also go well into trevor especially if you have good physical attack for florges, dragalge, hitting aurorus with fighting moves, and flinching clawitzer. generally too frail for star and don't have a clear niche into shauna though
ground: go to trevor and click eq. make sure to sub for counter. gets tricky vs everything else (watch out for team star orthworm) but vs trevor it's remarkably straightforward
rock: if you have a clickable rock slide you can fight trevor, though beware of slot 1's energy ball florges/surf raichu. most rocks should go neutral vs the fossil slot due to aero and aurorus being silly. if you have someone like carbink or stonjourner and are concerned about losing damage races, go beat up tierno's bird and get some extra damage in on rime.
dark: on paper these actually don't have any outright winning matchup spreads. looking at some of the mons you might be using like twave liepard fc persian-a and intimidate mabosstiff, these mons seem to do best in doubles especially vs star's high amount of physical attackers. as always you will have to leverage the mon to get decent consistent damage against the starmobiles.
poison: very very strong vs shauna fairy, but you get owned by diamond furfou/extrasensory gren/delphox so proceed with caution. have a ground resist for furfrou if nothing else. i observed that black sludge poisons are really annoying to kill for star cause they sit around poisoning everyone so that's another idea. of course you can always try your luck vs tierno.
flying: near-ideal matchup into tierno grasses. even someone like pidgeot or ledian will hold their own here.
steel: hard to say as there are many steels that do different things, but nearly always you should be good into trevor, you do well vs the fossils and even better into florges/dragalge if those are rolled. corrosion from dragalge is really not scary you can just switch out. if you have a double weakness like scizor or bastiodon you can consider enigma berry to increase survivability. these are surprisingly bad into shauna as you own the fairy but lose to everyone else.
ghost: if you have wisp go fight star. otherwise go to any singles sim, send out your ghost with second order, and click curse s1 so they have to take all 60 damage. curse is so good dude
ice: snow boosts your defense to surprisingly good levels vs star so if you have counterplay to schedar you're literally bing chilling. if you don't like this idea you can go fight shauna slot 1, even if you are physical you should do fine vs furfrou, and most ices also do decent vs gren
dragon: if you're special fight shauna and try to fish chesnaught delphox or furfrou (even noivern can beat all of these except specifically furfrou-star). if you're physical i don't even know who you're using, there are no bad physical dragons. the only arguable exception is cyclizar, who still does well into shauna thanks to regen + stab facade beating delphox if they burn you.
fairy: generally these are too physically frail to handle star or trevor's fossils. almost definitely best into shauna, your only outright bad matchup is delphox (and hilariously goodra if you have no play rough and can't take sludge bombs). greninja can't hit you super effectively outside of toxic combos, you own chesnaught.
alright thats all i got