Talk to your school careers person or whoever you think is best for information. The careers person at my school basically took care of it for me, helping out with my registration and applying for a centre. The usual place to do it is Scotch, but register quickly because they fill up pretty quickly...I had to go to Haileybury in Keyborough. The collegeboard website has pretty much all the information on how to apply to each university, what to write in essays etc.
Also (if you didn't know), if you plan to get into a decent university like an Ivy League, you'll have to prepare for SAT II Subject Tests so remember that you'll have to include that in your study plans for the year.
DONT SIT THE SAT ..... unless you are prepared to clog your inbox full of annoying offers from random colleges across the US
LOL it's so true...I get mail for this college that caters for students wishing to leave highschool early... it's pretty funny.