Have you plunged it? There is most likely an obstruction. ARe any other drains backing up? Test this by filling a basement washer tub or kitchen sink all the way up, then drain. If they drain OK, then the blockage may be between the problematic toilet and the Main Line in the basement (or whatever.)
You may have to remove the toilet to check this out. If you're not comforable w/doing this, have a pro do it. It's not that hard to do but you have to be aware of a few things BEFORE you start. If you want to know how to remove and re-install a toilet, post up again. In the meantime, 'test' out the problem with a plunger and by draining water.