Try not to cut anyone off.
Iv been doing iea for a while and it's a lot of fun! But you are on a horse you don't know so make sure you watch the horse go as many times as you can. If the horse seems like it's going to be hard don't get nervous talk to the horses own and your trainer to learn some tips and tricks! Have fun it's a great time!
Goodluck! I'd suggest that you watch the warm ups and rounds before yours so you are familiar with the horses. Sit the canter in flat classes, half-seat at the canter in the over fences class. Really know what distance you want to get ahead of time.
Ride the horse that's under you, not the horse you wish you had. Be nice to the handlers because if you're nice, they'll usually tell you more about the horse than if you were rude or standoff-ish.