I am going to resist the temptation to merge this into the other thread as I think there are a lot of players in your situation.
1. Don't be nervous. Pokemon is a fun event, and if you have prepared, just try and stay focused and do the best you can do.
2. Have a good time. Regionals are usually more than just a Pokemon tourney. Try to meet new people and hang out with them before and during the event.
3. Have a plan. Are you travelling with friends? With parents? Make sure to know where you are meeting your "group" so that you have a home base. Make sure you all have lunch taken care of, and bring along snacks.
4. Get some rest. A regional is like a big test at school. If you get a good night sleep the night or 2 before the event, you are going to be rested and confident. Lack of sleep makes people JITTERY.
5. Be prepared. Bring your decklist, damage counters, status markers (burn/poison) and the like.
6. Be consolidated. Don't take your whole collection into the event to make trades. Try to limit yourself to what you can toss in a backpack and keep with you, and KEEP IT WITH YOU.
7. Be a good sport. Win or lose, there is always next round.
8. Have fun.
Vince