Stay away from that hot girl that you'll fall for your meet senior year, she's trouble, I'm warning me. Seriously don't take the donut the first time you meet her, and don't let her in, she'll only dissappoint and hurt you.
First and foremost: Don't despair! I know your primary emotions these days as you navigate the social straits of boarding school are loneliness, self-consciousness, humiliation, anger, envy, contempt and wistfulness, but when you comfort yourself with the thought that a better life awaits you as a grown-up, you're absolutely right. Your life will only get better from now on.
Go ahead and date the cute ISFP druggie you'll meet soon - and yes, you can even go ahead and win him back when he dumps you nine months into the relationship even though your ENTP friend will tell you that it's a sign that your self-esteem is shot. However, do not get mad at and start drifting away from the ENTP for telling you this; he means well. Instead, tell him that he's right but that, for you, it beats playing EverQuest all day like he does.
Don't wait until you're 21 to start eating healthily and working out; do it as soon as you leave boarding school. (Between the food they serve and your current need for comfort, I'm not even going to try to tell you to start now. ) Also, it's good that you're working on your wardrobe, but don't take inspiration from what other women look good in unless they have the same body type as you. Oh, and definitely don't wait until you're 25 to ask a saleswoman for help in figuring out your bra size.
You can stop taking your mother seriously any day now; you'll cut contact with her at age 20 anyway. And keep listening to your older brother; you will go far under his wing, but don't ever actually outsource your judgment to him (as you'll do, and regret doing, a couple of times in your early twenties).
... And no, Ni is in fact not always right! You have no idea what I'm talking about, but that's for the best, because I don't want to rob you of your faith in your Ni just yet; you'll need it to make some difficult, but wise decisions in your late teens and early twenties. I just can't resist chuckling at the arrogance growing within you - even though I know all too well that such patronization only strengthens it...