Hmm 12.5 or so...
You'll try to explain to your mother why you're depressed, why you get bullied and why you hate life, she will call you a liar, it will go horribly and you will think about just killing yourself (but you won't). She won't ever get it, she won't ever accept it, yes it sucks, but hang in there...
You'll make a few friends on first day of high school, majority of them end up being assholes who keep you around for a bit to pick on you and fuck up the one good friendship you had. Don't lie or try to be someone else, don't try to be friends with anyone you think is cool, don't listen to fuckwits who end up ruining your friendship with Sarah by complicating it. Just go with it, keep hanging out with the people everyone tells you are dorks and enjoy it, anyone else you could be friends with just treat you like shit and you never really see again after high school.
In like 6 months you will however nearly die, you'll black out but you'll be fine... By which I mean you spend 2 weeks in hospital, 2 months on high doses of codeine, you will drop to 40kg thanks to not being able to keep down solid food and miss over a term of school.
In year 10 don't play indoor soccer, you will probably dislocate your knee and it completely ruins any hope you have of playing sport at a decent level again.
Don't give up Basketball, stop being an arrogant shithead who can't accept that he's too short to be play as a forward anymore, you realise later you're better at being a point guard anyway, you just need to learn to adapt.
Don't listen to your mother, ever, don't tell her anything, ever... She doesn't have your best interests, she ruins your life.
Don't bother putting in all the effort to get awesome grades early, your parents just keep shifting the goal posts and you end up burning out by year 10.
Don't care about what others think when it comes time to pick a direction and think about university, they never approve unless you do what they want you to do, if you do what they want you will be miserable and fail anyway.
Don't take a gap year, don't be a afraid to take risks, just get out of home ASAP.
Fucking just be yourself, you spend the next 10 years trying to be who everyone else wants you to be and it doesn't work and you will look back on these years and want to slap yourself for being such a dumbass.