Warm up is very personal. I usually do a pre-ride lap (about 5-6 miles) and push a few hills or do some accelerations (3-4 for 10-20 seconds). For me, a warm up is to prime the body to race without inducing fatigue. That said I’ve seen some high school teams do all out sprints, 5 min reps at threshold, etc.
There’s an old thread here on openers. Reading through you can see that everyone is a bit different:
As for what to bring… if you’re traveling to these races don’t forget dry clothes, snacks and maybe a towel for after the race.