depends on the competition. If you're talking high school I would probably recommend either defensive back or wide receiver if you're tall, or runningback or linebacker if youre shorter. Your coach shouldn't have a problem putting you where you can play your best.
If your fast: cornerback or wide receiver
Anyone you choose! - Most coaches would have you on the line
Punter or placekicker. No speed required.
In 7th grade you would probably be an offensive or defensive lineman.
cornerback or safety
at that height, probably better to play safety, too short for cornerback.
wide receiver if ur fast...goodhands. safety if not maybe cornerback