It is perfectly natural to get nervous before your first tournament since you don't know what to expect. Knowing what to expect will help you be better prepared and ease your tension. Read this post about what to expect during your first bjj competition then read my other post with advice for winning your first bjj toutnament. Most larger tournaments will let you weigh in the night before. If you aren't sure you will make weight and need to cut water weight, I recommend weighing in the night before. Naga is one of the larger tournaments and they have events all across the United States. Their scales are usually calibrated so that you are 2+ pounds lighter than you actually are.
Day of the tournament it will be very crowded in the gymnasium and very loud. Since men out number women by a large margin you can actually expect to have lines for the men's bathroom and not the women's bathroom. Children's divisions are usually first, followed by the women's divisions with men last. Tournaments are always running late so expect to wait around for a while. Typically you will start 2+ hours after the time advertised on the schedule. You need to pay attention and listen for your division to be called. It can be very difficult to hear so I recommend checking with the organizers frequently to see if it is your turn. Bring small snacks to keep your energy up. You can't really have a large meal because you don't know when your match will be but you could be waiting a long time until you start.
When you get on the mat you will need to take off your shoes. When you go to the bathroom you will need to put shoes or flip flops on so that you don't track bacteria from the bathroom onto the mat. Be sure to warm up and stretch a few minutes before you start so you are loose. Remember to breath and don't panic. If possible breath in through your noes, it is more efficient and slows your breathing.
Naga and Grapplers Quest and notorious for running late. The Good Fight is a smaller tournament that has a money back guarantee if they are running too late, so it could be a good alternative.
Video of first No-Gi Jiu Jitsu Tournament
Video of first Gi Jiu Jitsu Tournament