it depends if you are good at catching if you are I would say fullback
The best position is fullback or winger, in which both positions requries the players to be fast.
fly half, scrum half or full back
Its appears a utility back but a good coach will advise you of the best position at this time
At your height i would reccomend scrum half or fullback you are simply not tall enough to be able to fend off anywhere else due to your height it is not un-common for 6 foot 7 second row's!
number 8 or centre may be a start as you are still growing at this age and therefore you may change body shape which in turn might drive the position you play