Yay congratulations!! Riding lessons are a ton of fun, and the people there will quickly become your "second family". It's okay to be a little nervous. Most barn owners/instructors are mindful of this, seeing as they deal with new riders all the time. Everybody starts somewhere!
A couple of tips you may or may not already know:
~ Wear pants and close-toed shoes to your first lesson. If you don't already have riding pants and boots, you will eventually have to get them, but most instructors don't mind if you show up in jeans and sneakers for your first time.
~ Bring a water bottle or two! You're going to get thirsty, and it's always polite to bring your own drinks. Although, a lot of barns I've been to have water tanks that are meant for all of the riders, but it's better to be prepared in case there isn't one.
~ I think this was already covered thoroughly, but I'll repeat it because it's important. Don't be afraid to ask questions! Also, during your lesson, don't feel bad or ashamed if the instructor corrects you or if you keep doing the same thing wrong. That's what riding lessons are - you're going to make the same mistakes over and over again, and it's the instructor's job to remind you of them every time you make them. There's nothing bad or shameful about it - it's all part of learning.
~ Most importantly, have fun. Riding horses is an amazing privilege. I think it's much more than just a hobby; it will shape and mold you as a person. You will learn responsibility, respect, and hard work.