Me aged 14 at secondary school.
1) Spend as much time as you can with family. Pretty self explanatory, but during my teens I avoided this a lot. It's one of my biggest regrets. Time is of the essence and it's time you won't get back. Enjoy your family while you can.
2) Stop being so god damn moody! I've always been a bit of an 'angry Anderson' as mum would call me. But I wish I'd spent more time enjoying some of the amazing experiences I've had rather than being a moody little rat bag.
3) It's ok to be anxious, but don't let it rule your life. Too much of my life has been wasted because I've let anxiety overrule doing something. I've come to the conclusion of if you're with the right people they won't care if you have a panic attack and be sick. They'll probably just laugh and get over it. And you will too.
4) Stay away from lambrini... Cracking your tooth and splitting your lip open will be one of your worst teenaged memories. And the boy you were trying to impress doing so will tell you he thinks your a prat and never speak to you again.
5) Alevels are not the be all and end all. You'll do shit. You'll get over it.
6) It's ok to enjoy reading, science, poetry and history. You'll be a font of random facts when you're older and who cares what other people think anyway? Bringing me to number...
7) Don't give a shit about what people think about you. You're just like marmite and the people who care about you wouldn't have you any other way. Annoying as you might be.
8) You'll never understand boys. And that is normal. They don't understand you either.
9) Don't tell somebody you love them when you don't. It makes things awkward and a lot more serious than you'd actually really care for.
10) You will be called fat more times than you're own name (ok maybe an exaggeration) and yes you will have a weight problem, it's all part of this thing called becoming an adult. You make bad food choices. One day it will click and things will change. And everyone who called you fat can take a big fat middle finger and a f*ck you.
Finally, you'll turn out a relatively average human. So don't worry. Enjoy the present.