Well, try not to use it too much for a day or two. You can wrap it in an ace bandage if you've got one, or they sell them at Wallgreens. Take a pain reliever and ice it a bit. Keep it elevated when you can, and if it's not better in a day or two, see a doctor.
If you are a teen, make sure your parents know that you hurt it. They can help you with pain relief and a bandage, and of course if it's not better in a day or two you'd have to tell them anyway. It's definitely better to tell them now, than to try to explain to them later that it's already been hurting for days and you want to go to the doctor NOW. ;)
The pain could be caused by a pulled muscle or even a fractured wrist bone. The only way of establishing exactly what is wrong is by having the wrist examined or X-rayed by a doctor for the right diagnosis and treatment.