Make sure you wash the area thoroughly. Something like peroxide would definitely sterilize it, but will also cause significant pain. You may want to simply wash it the best you can with warm water and a clean cloth. If it's still bleeding, you should sit with your dog for awhile and provide medium pressure with a clean bandage or cloth to stem the flow. These types of injuries become infected easily. You may need to use an antibiotic ointment, and possibly dressings, to keep the area protected while it heals. If any pink or red areas or swelling develop, or if the area becomes very warm to the touch, you need to see a vet for infection.