You don't need to know any skills before you start and in general you shouldn't try to teach yourself. Not only because you can get hurt but also because you will more than likely end up training poor technique and once you build muscle memory if that it can actually make it much harder to learn to learn to do the moves well than if you were starting from scratch. Most coaches would prefer you start with a blank slate.
You can do conditioning to prepare your body to learn gymnastics skills by improving your strength and flexibility though. This means stretching and body-weight exercises. It would also be good to improve your general fitness, skipping is a great way to do that. The only skill you should think about working on before you start is handstand. You can start by walking your feet up a wall if you aren't comfortable to kick-up against a wall. Otherwise leave it to the coaches to teach you new skills.
In no particular order
Floor: forwards roll, backwards roll, back bend, cartwheel, handstand, straddle roll, leaps and turns, roundoffs, or anything else you can learn by yourself
Bars: pullover, cast, glide swing, chin ups, and pull ups would all be helpful to know
Vault: for level one, it's very simple. I think you just need to know how to do a squat on onto the table and jump off
Beam: as long as you can balance on it, you should be fine.
You should also learn what a tuck, straddle, pike and straight jump are. You should also learn the difference between point and flex. You should also learn what releve, coupe, and posse are.
Although, you don't really need to know all of these skills and terms are because they should teach you.
Back bend(sometimes kick over it helps), cartwheel, hand stand, round off( with good rebound) and start stretching every day to improve flexibility it will help
Cartwheel, handstand, backbend,roundoff
