At 14 or 15, I wouldn't be too concerned about weight. Give it a few more years, your metabolism will slow down and you'll start to bulk up. I was 6 foot 5 and 180 lbs in high school, and though I grew another two inches, by second year university I was 230 lbs. Granted, it was fat and I'm back down to about 195-200 lbs now. But yeah, I wouldn't be too concerned. Just get yourself into a good workout routine. I would recommend staying away from weights though. I've read some articles that say exercise at too young of an age can actually be bad for you, as your body is still growing. Just my thoughts.