The best way to stop bleeding is by applying pressure to the area with a clean cloth or medical gauze. Holding an ice pack on the area outside can reduce bleeding, and limit swelling and bruising. Wrap the ice pack in cloth, so the ice doesn't touch your skin directly.
If there is a lot of bleeding, or the bleeding continues past 10–15 minutes, go to an emergency clinic or your dentist immediately, as stitches may be required. Whenever you have any dental injury, you should ultimately seek the advice of your dentist.