If I were you Id get weights, a lot of them and then go on a hardcore bulk diet since you're still young and skinny, that will build up your mass and muscle, after a bulk which should last for about 2 years go on a cut for 6 or so months, doing weights about 5 days a week during both a bulk and cut. You'd be ripped by the age of 17-18
Doing sit ups will only work your upper abs, and push ups will work your chest, and a little on your triceps depending on how you're doing it. You're not really doing any shoulder, back or arm workouts