First reaction is usually to hold your head back, but this could lead to choking. You should lean forward and hold your nose for about 5-10 minutes. Gently release, and see if the bleeding has stopped or lessened. If not, gently hold for another 5-10 minutes. Most importantly, remain calm. The sight of all that blood can be frightening, but stay calm. If after this, your nose continues to bleed, you should consult your doctor.
Nose bleeds usually are not serious. They can be cause by several things such as dry nose, allergies, etc.