I did my first race last fall and I completely understand! Here's what I didn't know...
It's tricky to understand how the whole thing works if you've never done it before. If regatta racing is completely new to you it is VERY helpful to watch a couple of races from the committee boat or crew with someone that knows the ropes before you jump in. I my case I dove right in, and got really confused, and figured it all out after a DNF or two.
While that's a quick way to learn, my biggest concern was not interfering with the experienced folks. On my first race I basically sailed around the course but stayed behind and outside all the other boats to watch how it was done and not interfere. That can be difficult to do in a multi-class regatta with lots of boats on the water and staggered starts.
In addition to the good advice in the post above, I'd add a couple of other things...
Tell the RC you are a first timer and ask them to explain how the Starting Sequence, Signal Flags, and Course numbers will work. It can be very hard to understand if you are trying to figure it out on the water and it's easy to miss a course change. (For example: I DNF'ed a race because i didn't realize they added an additional leg to the course on the last race of the day.) It's also not a bad idea to debrief with the RC after the race. They will point out things you did wrong that you probably never realized. (Like getting disqualified for crossing the start line in the middle of the race)
Get one of these: Learn it, and put it on your boat - Best $4.50 investment you will make. - http://www.hcana.hobieclass.com/default.asp?Page=9276
Play this game: http://game.finckh.net/indexe.htm
until you have mastered the "basic" level
It's a little daunting the first time out but it also REALLY fun. Focus on learning the ropes, mastering technique, and staying in the race. Don't worry about where you finish.
...and it wouldn't hurt to practice. Go tack/jibe your boat about a million times before you get there. Your ability to stop, start, stand still, and tack quickly will keep you in the race. Practice sailing up wind efficiently. Not something recreational Hobie sailors don't do much of. This is where the old guys will blow past you on the course.