First off, you haven't already taken the SAT?
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I know I need to do good, >>
second, it is "I know I need to do well" you should start there with proper English
As for how to get motivated, I would go out on Friday night and live it up (not too much you will need to get up on saturday) get up at like 8:30 a.m. saturday and start going over your algebra and geometry skills, set goals for yourself, that should help you get motivated, once you accomplish one goal, set another, you should find that you like succeeding
Take breaks too, if you feel like you aren't doing anything, or you are day dreaming, take a 15 minute break, kick back, listen to the radio call a friend, nef on ATOT, then get back to it, you should also try to find books that you like to read, as they will dramatically improve your English score.