Don't worry, they'll teach you everything you need to know. I have worked for this type of company and they expect green guys to be basically useless the first couple days as you get into it.

Best advice I could give is make sure not to step or put anything on top of cords and hoses. Stay out of other peoples way, staying out of the damn way is the first skill you will learn. Watch out for peoples drinks, if you spill somebody's coffee in the morning expect hell the rest of the day.

Get a set of tool bags too, some good cheap brands are McQuire Nicholas, Husky, and Dead-On. Get a trim hammer, 16 ounce with smooth face is pretty standard. Tape Measure, 25 foot, Dewalt or Stanley Fat Max are both good, don't buy a Milwaukee, they don't do well when you are trying to pull across a face. Get a speed square, swanson for rips. Get a cat's claw nail puller, Fatmax, Dewalt, and Estwing all make decent ones, Estwing is my preference though they're a bit more expensive. Get a chalk like and fill it with #2 chalk, don't get 3 or 4 chalk for finish work it sticks to everything. Also, make sure your chalk like has 2:1 or 3:1 gearing, they cost a dollar more and reel in much faster. Utility knife, get a decent one, spend three bucks for a stanley they last forever. My Stanley knife and Swanson speed square are the only tools that have lasted the years, they cost 12 bucks combined and hold up spectacularly. Lastly buy a nail set, or a couple of them with different heads.

Other tools you can mix in but probably won't use as much are torpedo level, chisel, adjustable wrench or small vice grips, screwdriver with changeable bits, wonderbar which is a tiny pry bar extremely useful but not required.

Basically pay attention, ask questions, and don't do anything you aren't sure you are supposed to. Also be super super super gentle to start, everything you install as a trim carpenter is finish produce, the less you bang it up with hammer and bend nails over into it the less time it'll take to finish it properly.

Good luck