In no order of importance:
Start saving now!
Go to the GP, stop being embarrassed and get your various gyne problems sorted before you end up in your 30?s with severe fertility problems as well as other linked issues. What you?re experiencing is not normal, you are not over-reacting/ being hysterical do not let some old git persuade you that you?re being a drama queen, every woman is not experiencing the same issues. Stop being an idiot, tell Mum she will go ape shit and push for you to be referred to the hospital, stop trying to be over grown up you don?t know everything!
The ?popular? girls don?t hate you, well they do but only because you intimidate them. (it's not your fault that the school skipped you ahead even though you're 'too' lazy to spell only (one teacher believed in dyslexia, you should have paid more attention to her))
Queen Bee of the school who seems to have made it her mission to make your life a misery will turn down a vgood uni place, marry her first boy friend at 18, who will continue to cheat on her as he has done from the first day they meet, will never leave the country even on a cheapie holiday and secretly envy?s you (you will find all this out in your 30?s when she drunkenly accosts when you visit home, and although you at 16 you would do a victory dance, at 30 you will feel sad for her, and her vomit will come out of your shirt, but really just throw it out) 
Be friends with the boys in your year, you can be just friends, you are not a whore or a slut for doing this; in 10 years time when some of them are your best friends you will regret missing those 10 years.
That older boy, you know who I mean, is not being romantic, jealously is never romantic, he is a stalker, take it seriously DO NOT BE flattered.
Buy shares in Apple
Save, money is freedom
Learn to drive; again doing this at 30 is a bitch
Realise that just because you?ve been friends since you were 4 does not mean you have to continue to be friends dump her now it will save you thousands of pounds and hundreds and hundreds of hours of whining phone calls .
Stick with the piano lessons, you?ll never be great, but you will always enjoy it.
Learn to touch type
Ignore anyone who tells you that your dyslexia will stop you going to uni, spell-check is just around the corner.
Take every opportunity to travel while you have no responsibilities, get a passport now.
Try to develop an adult relationship with your parents, they are amazing people, you?ll realise this once you stop being a brat, and you?ll have wasted 15 years.
Your just older sister is a bitch, it isn?t you, stop blaming yourself, you are 2 very different people, and you can love and support her without being joined at the hip.
Always buy good shoes
Stop being afraid of getting your eyebrows waxed, it will change how you see yourself, really, oh and the purple eye shadow. Throw it out now.
Vodka is bad evil gut rotting lovely delicious horrible stuff, never drink it. it will make you think you are smarter than you are, you will make bad decisions under its influence.
Learn to walk in high heels now, take lessons from your Mum she is a master.
Oh God there is so much more but I?m hijacking aren?t I