In no particular order:
1) It's okay to be lonely sometimes. Everyone feels lonely sometimes. It's normal. When you feel lonely, tell your brain, "It's okay that I'm lonely right now. It doesn't mean that I'm sad or lacking, it just means I feel lonely right now." Don't worry about it. It won't last.
2) It's okay to be alone. Try not to be alone all the time, though. People are pretty nice and good people.
3) Boys are more trouble than they're worth when you're young. Study hard, use all that great young brainpower, and worry about boys after college. You've got loads of time, and it'll be worse for you in the long run to start with boys too soon- there's no such thing as starting "too late." There's *always* time.
4) Only wear comfortable shoes.
5) You can tell people to clarify when you don't understand something. They won't think you're stupid, and once they explain it, you'll understand it forever. If you pretend you get it, but don't, it becomes harder and harder to learn it *cough* multiplication tables *cough* as you get older. I promise, they won't think you're dumb. Just ask.
6) You don't have to be good at everything right away. Seriously. It's totally normal to be actually quite lousy at lots of things. Every single person has at least one thing they're terrible at, one subject that they're utterly clueless about- really, honest, and truly- everyone is awful at loads of things, and it's okay. Trying your best is way more important. Try really hard and stick with it. Even if you still aren't a good dancer, artist, or whatever, that's okay. Don't just give up when it gets hard, but don't beat yourself up if it's never perfect.
7) You don't have to be an expert on everything. Really, people don't care if you don't have an opinion on something. If everyone's talking about, say, Middle Eastern politics, and you're not up on them, it really is okay to say, "This is interesting! I'm not so up on this- could you tell me more?" You don't have to speedy-quick think of all the things you know about the Middle East and formulate a very firm stance then and there. I promise. No one will think ill of you.
8) Inside your head, *feel* like you don't care what anyone thinks, but don't *act* like you don't care what anyone thinks. If someone thinks you're dressed funny, boo on them, but don't dress in a way that would cause the average person to think you're rude and disrespectful. Don't roll your eyes at people behind their backs or in front of their backs- no eyerolling, period. Well, okay, just a little. Don't tell people off, even if they've slighted you. Always be polite and carry yourself well. Don't say mean things about people. You're going to be tall, so stand up straight. People will treat you well if you act like you care what they think of you- show them that you want to be taken seriously, and you will be far more well treated than if you carry yourself like a thoughtless jerk. (You can still always wear sneakers, though. I'm okay with that.)
9) You can read books all day, but not every day. Lots and lots of days, though.
10) You don't *have* to like to read. I hope you will, but if you don't, it's okay. Perfectly smart, amazing, educated people don't really love reading for fun.
11) Most people really are pretty okay. Give them latitude before you judge them- people usually don't *intend* to be unkind.
12) I love you more than anything, and no matter what happens in your lives, you can come to me. I'm a pretty okay kinda guy, for a mom, and you can trust me.
postscript: This obviously should actually be titled Some Things I Wish My Parents Had Taught Me, or, let's be honest, Some Things My Parents Tried To Teach Me But I Was Arrogant And Foolish Thus Ignored And Also Some Things No One Ever Taught Me.