What do I do about dizziness?

At first it was only on rare occasions, now dizzy spells seem to overtake me more frequently. For most cases, it is when I'm sitting down,then suddenly stand up, and start to walk. It is at that time that I feel extremely dizzy, have a hard time standing, my vision blurs, my head has a lot of pressure, and I can't stay focused. Sometimes it lasts for about five seconds. Other times up to a minute or two. But what I don't understand is I'm only a teenager. What could possibly be wrong with me? Help! (And thanks in advance.)