How do i increase my stamina?

Well it depends: My first line of questioning would be to evaluate at your age what might be interfering with your stamina. Of course there a whole host of possible reasons but at your age to be asking the question makes me think that at the very least a basic work up is indicated. More likely than not there are correctable reasons for your concerns about a lack of stamina.

Healthy Living: Exercise regularly, eat a high protein, low sugar diet, and get plenty of good restful sleep.