Well, you're clearly not big enough yet to play O or D line (not that you would want to), so that narrows it down a little.
First question, do you want to risk getting hit alot (maybe hurt) for the glory of making touchdowns? if yes then keep Offense an option, if not.... stick to defense.
Second question, do you like to hit people? And give up 'some' of the possible touchdown glory for making the 'Big Hit', if so, then defense is for you.
For your age, you are probably average for linebacker or running back size, and both are a lot of fun! Work on your speed and quickness... and the size will come.