1) Finish school ASAP in something marketable, you can always go back to study something you love once you have a proper job.
2) Enjoy summer vacations while you can! Grown ups don't get as much time off, and you'll miss all those weeks of fun and waking up late
3) You're so much prettier and cuter than you think, don't let anyone tell you otherwise. One day, you'll be looking back at how you look right now and regretting how bad you felt about yourself.
4) Go to as many concerts as you can. Ticket prices keep going up, and the bands you loved that always had general admission tickets? They'll be playing in arenas with assigned seats one day, and you'll never be that close to your music idols again.
5) Don't be so trusting of boyfriends. The bad ones know just how to manipulate people, and they won't spare you either. Be careful with your heart and just have fun.
6) Life will get better! It can feel so depressing and hopeless when you're young, but it will be better when you're older, I promise.
7) Don't be so judgmental of people that like different music than you, or that dress differently than you do. Superficial stuff like that is the least important thing about people, and you're going to miss out on so many cool people if you judge them on stupid things.