Strange headache when I move my head?

I'm 15 and three months, 6'4" tall, and at a healthy body weight. I don't drink water too much but I drink lots of milk. My diet has been pretty healthy and I haven't eaten out in a long time. I don't get lots of physical activity, about 20 minutes each day. I was on the computer for eight hours yesterday working on a project. When I got off I had a slight headache. This morning I woke up and felt fine but when I got up I had a dull pain in the center of my head. When I move my head I feel the pain but when it is still or when I'm sitting down not moving it doesn't hurt anymore. I am worried and want to know what I can do. I'm going to drink lots of water because it might be dehydration, but is there anything else I should do?