If your grades and SAT scores aren’t good enough to make the cut, your ECs (unless you’re a recruited athlete) will not increase your chances. I can’t really tell you which ECs to join without knowing your interests, as I’d never recommend that somebody do an activity they hate just to look good for colleges. Also, it’s better to be very involved in a few clubs than be in a lot just for show. Start by actively participating in library club and look for others to do the same in while the school year is still new.