Do you know where you want to go to college? A lot of schools don't even have a Greek system... some have only local sororities... some have a handful of nationals... some have a lot. My advice: First figure out where you want to go to school. Academics is always the first priority. Once you've been accepted to a given college you're interested in attending, examine the Greek system, seek out the recommendations you need (if any - this varies from region to region) and go from there. And keep your grades up - resist "senioritis" - because if your college has rush at the start of freshman year, they will go by HS grades, and you want to be sure you qualify.
Good luck!