What to do to gain height?

Eat more: The most sure fire way to gain weight is to eat more. What i would recommend is exercising with increase in a healthy diet to build muscle to gain weight.

See below.: Unless you began puberty late, you are not likely to have any further significant growth spurts as a 21 year old male. Maximize your height through proper posture & stretching - however this will not cause you to grow taller.