I am 17. I am 198 cm. tall and I am thin and athletic.
I have just started doing:
25 weight lifts each arm (I increase the amount of weight lifts I do after every 7 days)
When I have time to go to a Gym. I do shoulder presses, chest presses and bench presses. And 10 different stretching exercises for improving my flexibility.
Start with at least 200-300 situps a day.