Five priceless pieces of advice to my 15 year old self (copied from another blog):
1. Use your head when it comes to boys. They're bound to cause you a lot of grief; not heartbreak, but annoyance. Don't spend too much time with just one. On the flip side, it's best not to date boys who are friends, or belong to the same group or 'clique'. They'll always take each other's side and make you feel like the bad person. And though it may be funny at the time, and may get you some attention, never share personal letters with others, especially the boy's friends. Not a good idea. It's better to be friends with boys at this age. Relationships now really mean nothing in the long run.
2. Don't make fun of others, especially not to their face. Understand that everyone has their downfalls, including yourself. Don't use those things against them to make yourself feel better. Do your best to accept them for who they are, and always laugh WITH them, not AT them.
3. When a friend is in need, don't turn your back on her, even if you have no idea how to handle the situation. She needs you, not your advice; she has others for that. Be a friend. Don't say anything. Just sit with her, hug her, or let her cry on your shoulder. You never know how your support will affect a person over the course of her life. The most important thing to remember is being there for a friend is never a bad thing. Don't be scared of new situations. You'll never do the wrong thing, say the wrong thing, or piss her off.
4. Don't be afraid to ask boys out. If you wait for them to ask you, you're limiting yourself to a certain selection of them, which isn't always a good thing. So what if they say no? It won't make you a nerd. If anything, they'll feel great that you asked in the first place, and you'll be that much cooler. In fact, don't be afraid of people in general. If you want to say something to someone, say it (within reason, of course). Talk to cheerleaders, burnouts, gay kids, jocks, athletes, punks, and nerds. They might have more in common with you than you think. You can never have enough friends in high school and college.
5. Don't go to college with a bunch of friends. It might be fun, but it's safe, and that will get you nowhere. You need to be forced to make friends, be forced to find yourself. Go somewhere far away, somewhere you'd never think to live. This might be your only chance to get out of Missouri for a little while. You'll have plenty of time to see your high school friends in St. Louis during holidays and breaks. And make it a smaller school. That way it won't be so easy to slip between the cracks.
It's crazy to think of what my life might be like now if I'd had this advice back then. I know it'd be totally different. I'd have still made plenty of mistakes, I'm sure.