Fast enough so that you're not getting smacked around. Seriously, velocity shouldn't be a worry, just work at throwing FBs at a velocity that you can control well and mixing it well with your other pitches.
You throw a curveball/screwball and you're 13? First off, don't throw a screwball. Not only are you way too young to handle its stress, but even if you were old enough, you'd have too much stress on your arm. You're better off not pitching it at all.
You shouldn't throw a curveball right now either. Its bad for your arm at your age. Wait till you're 14-15.