Liam

You still very young i wouldn't worry. Probably got a good bit of growing left in ya.

I wouldn't bother with any weights until your a few years older all i can suggest do as much walking / running / cycling as possible.

I find that now i am training every time i play football with my m8s (just a mess about) i try harder and push my self... this way it gives me a good workout without the bordom of running on a machine. I suggest you try something like that, or even walking the dog once a day.... you'll be surpised the effect something little like that can have one your body when you've never done it before