I was scared with my first race, too.
But there is nothing to be scared of. Most people are very friendly and the race atmosphere is fun, exciting and high-energy. I don't know particularly about trail races - I've only raced on pavement, but a few rookie mistakes I made with my first race....
- don't start out too fast in all of your excitement/nerves. You'll tire quickly and the end result is usually not pretty.
- Try to relax and remind yourself that you're only there to finish the race. I tend to get caught in how fast other people are running and get competitive. Competitiveness is a good thing when it doesn't ruin how you feel about yourself and your accomplishment. Running by itself is tough; running a race is rewarding and you should be so proud of yourself for doing it!
- Don't try anything new on race day (new shoes, new foods, etc.) Go with what has been working in terms of meals and rest, and you should be able to avoid any umm gastro-intestinal issues.
- For a 5k I would not carb load the night before. You're just asking for trouble lol!
- Have fun.
Recognize your accomplishment and pat yourself on the back for doing something good for your body!