Position depends on agility and speed as well as physical dimension. Most 15 year olds with your size will be moved to the second row and if fast - number 8. This is because you are classed as tall for your age group therefore its wise to take advantage of that fact.
You would probably be full back or wing
Ball boy or team mascot
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