If you are generally athletic, you will do better if you play relative to your speed, size, weight, compared to other people of the same age. If you think you're fast, play defense, maybe a corner back or a safety - it's pretty fun. If you think you're strong and have good balance, be a blocker and play one of the lines. If you think you're tall and can catch well (you should be fast too) play wide receiver. I would not recommend running back since it requires all of the above qualities and you are subject to injury. Being a QB is hard too since so much of the offense relies on you. Maybe learn the positions and do whatever is most fun!
wide receiver on offense or cornerback on defense