I know what thats like. Ive always been tall, thin ectomorph body type. I saterted college at around 6' 2'' 150lbs. Three years later Im now up to around 180. Thats all from lifting weights. If you have the body type that cant put on fat then go to the gym, start a weight routine with the help of someone who knows proper technique. Stay determined and lift hard, but give yourself plenty of rest time between working the same muscle groups. Also, nutrition is key. Start eating a bigger breakfast, taking protein supplements and most importantly drink lots and lots of water. Try eating more often, optimally 5 or six times a day. Also, dont neglect lower body stuff when working out, like squats, leg press, etc. Lifting hard with your legs stimulates hormones and growth in all other areas. Youll get there.