GPA stands for "grade point average." Its basically a measure of your grades, averaged out into a specific number that's set on a scale from 0.0 (straight Fs) to 4.0 (straight As).
Well, word of advice: try to do as best you can on the SAT and try to get into the best college you can. There is truth to the assertion that undergrad is not as important as graduate school, but that doesn't mean that you should go to a crap college. It just means that ranking isn't as imports for undergrad, and that Ivy League wouldn't be worth it for undergrad if you got into other comparable schools. However, Ivy League for law school would def be worth it. Unless, of course, you got scholarships at other comparable law schools.
By the way, what is your GPA currently? That's an important indicator for college.