Don't think of it as $20 for 4 minutes of contest flying, it's more like admission to a 2 day show of flying and fun, with the added bonus that you get to participate.
Make sure your stuff is ready, check it over for loose hardware, good fuel lines, have spare lines, glow plugs and props, bring a back-up model if you have one.
Don't be shy, introduce yourself and find the other Beginners to meet who you'll be flying with. You can recruit a helper if you came alone, and offer to pit for any one who needs a hand.
And have fun- I was you a couple years ago, and I still crash on a regular basis - sometimes for the Judges in Beginner. It is still a ton of fun you can have for $20, and you usually get in on a raffle.