What are some easy exercises I can do to stay fit?

Everyday in school I am dull,even if I get 9-10 hours of sleep. I have no extracurriculars and no exercise, in the past I used to think that was not a problem. Now I realize everything. I was watching something on Shaolin Kung Fu and after finding out that since the monks in the past just copied and didn't train, they were not able to stay awake easily. I face this problem now. What can I do to be fit? I want to stay awake in school, I mainly want to stay awake during my ACT (I got a 26/29, I almost fell asleep both times by the time I got to reading, so I think if I can stay awake I can do a lot better). So please help, exercises can be 10 min to 1 hour. When I was a kid I ran super fast, I started playing video games when I was in 9th grade and didn't do any sports so that is where I got screwed. I went from 99 pounds (5'5 at the time) to 155 pounds (5'11). I would imagine that is somewhat dangerous for it to happen in 3 years. Anyways, I need some good tips to exercise to get back into shape, so please help :)