Good luck to your daughter. I think I was more nervous than my daughter for her first meet a couple month ago. Funny now it seems so long ago as we are almost done with our season. I think everyone gave you some great advice. I think the sleep is important and making sure you have water and a healthy snack. I give my dd some grapes in a baggie and some small pretzels. Don't think she ever ate during the meet, but it was good to have something to pop in her mouth as soon as it was over and they were waitting for awards which can take a long time. I would agree with the other moms, be supportive, tell her she did a good job, give plenty of hugs. My DD didn't really understand scores in the begining. Boy did SHE learn fast though. But it wasn't something we discussed until she was trying to qualify for sectionals and then states and then she would ask if she got a good score.
My daughter is not a good eater pre-meet. We try to get some cereal in her for early morning meets or a late breakfast for early afternoon meets. Just be prepared to hit the first McD's or other food place as soon as you leave the meet cause she will be hungrey! If you are bringing other kids, make sure you have a gameboy, book, snacks, drinks or something else for them to do...meets can get long for siblings & expensive for parents! Plus you will soon get tired of "meet" food, hot dogs, bagels, muffins, meatballs.
Bring yourself a pen to keep score, it's fun to keep track of everyones score and makes the time go by quickly as you wait for your dd to compete. Make sure camera/video is charged and remember no flash allowed. Take some nice pictures before the meet starts because afterwards you will not be able to nail them down for a decent picture & the hair-do will look nothing like it did 3 hours before. And hair...find out what hair your coach wants the girsl to have and start practicing. We started out with a bun that I found nice and easy and then ended up with a french braid. Not easy for a mom who had 2 boys first and is hairdo challenged. Make sure you have hair gel, hair spray, bobby pins, and GLITTER! I buy a couple cans after Halloween when it is always on sale. One more thing, pack a needle, thread and safety pins in your purse, leotards have been known to come undone at seems, it happened to us. And if your gymie has a bag throw in some bandaides, tissue and a roll of tape, someone always needs tape or a tissue.
Think that covers it. HAVE FUN!