When I stand up too fast i get lightheaded?

I am 13 years old. I'm a couch potato, I just sit down for 24/7 playing on the computer, I only get up to go to the bathroom, eat, and go to sleep. And it started in the beginning of the summer, when I get up really fast i get lightheaded and my vision goes fuzzy for a few seconds. It's not like I'm going to fall over or anything. What could it be?