There's 17 things I learned when I was 17:
1. People will promise to never leave you. They will. It's okay to be sad when they do.
2. It's always okay to cry. Always. Find a bathroom, burry your face in your pillow. Cry in the car. Cry when you need to.
3. Boys will flirt with you for a while and then ignore you. Then, they will flirt with you some more. It will be confusing. You have every right to stop putting up with it.
4. Pay attention to what people say when they're angry. When you make up and they tell you they didn't mean any of it, know that they did. Also know that they wish they didn't. Forgive them.
5. Never pretend to be someone you're not. If you don't like tea and classic novels, don't act like you do to impress people. If you don't want to wear leather jackets and combat boots, don't wear them to please someone else.
6. People will be mean to you: they will spread lies, call you names and talk about you behind your back. Eventually you will realize that it's petty and stupid and not worth your time. You'll be right. Move on with your life.
7. Your friends will not always be there for you. When you really need to talk, they will sometimes not want to hear ir. That's okay. Take a deep breath and remember all the times you felt the same way. Exhale.
8. You will wait and wait for your first kiss and your first date and your first relationship. The anticipation will kill you. You will keep trying to find the right person in everyone you meet. Relax. The best things happen unplanned.
9. Enjoy being young. Love that everything is spontaneous. As you get older, things become more and more scheduled out. Embrace the fact that you aren't there yet.
10. Tell people how you feel. It will be terrifying in some cases and gratifying in others. It will create relationships and ruin them. But speak you mind, even if your voice shakes, because your thoughts may never otherwise be heard. 
11. Sleep. If you go to bed late, sleep in. If you're still tired when you wake up, go back to bed. If you can't stay awake during the day, take a nap. Sleeping is a foolproof way of getting rid of your problems for a little while. Utilize it.
12. Talk to people. Talk to your sister about the guy she likes. Talk to your mom about her childhood. Talk to your dad about his favourite books. Talk to your grandparents about their families. Talk to your friends, talk to your pets, talk to the cute waitress at the restaurant. Learn things from them. Be inspired.
13. Always bring a sweater. Even if you think it won't be cold.
14. Try new things. Eat a new food, try a new kind of juice, switch up the way you dress. You never know what you might end up loving. Life can get boring. Mix it up a bit
15. Take care of yourself. Wash your hair with that good smelling soap you love. Eat fruits and vegetables. Drink lots of water. Go for long walks in pretty parks.
16. School is important. Try your hardest. If you don't get something, ask for help. Show your teachers that you're willing to work hard and when it comes time to apply to collegues, you'll be glad you did.
17. There will always be someone prettier, smarter, funnier or more popular than you. The beauty of it is that it isn't a competition.