How can I gain more muscle at 15 (read more details)?

I am a male. Almost a year ago I started working out 3 times a week. I work out on Monday, Wednesday, and Friday. I do push ups and sit ups on all those days. On Monday I do bench press and dead lifts, on Wednesday I do shoulder and leg exercises and on friday i do bicep and triceps exercises. In about a month I had much more developed biceps and I had developed a small chest, and I was developing abs slowly (no pack though). But ever since then I haven't grown at all, i am stronger, I can bench more and lift heavier dumbbells but I am not bigger, I want to gain size and I want to get cuts. I weigh 120lbs. Any help?