Don't let a game jam be a total measure of your skill. The only thing that will help you isn't doing a time-sensitive competition, but taking your time to familiarize yourself with the engine and code you're working with. Practice, practice some more, see if you enjoy it, then ask yourself whether you want to keep doing it.
With that said, Jams are pretty fun. Don't look at it as a competition, especially for your first one. Just think of this as an opportunity to create a finished product (Deadlines certainly help with that aspect at least, LOL)