I shut the door on my pit bulls tail, it is bleading and I don't know what to do?

Tails bleed a lot once injured and are very difficult to heal.

Short term solution: apply pressure with your fingers to try and create a blood clot. Do not bandage as if you do a bandage too tight your could cause further damage by restricting the blood flow too much.

Long term seek a veterinarians assistance. Tails some times do not heal or can take a long time as dogs continue to wag them.

If its still bleeding after 5 minutes from the time you injured the dog, take him to the vet.

You might want to ask this in the "Dogs" or "First Aid" forums. If the bleeding continues, I'd call an ambulance or an emergency vet service.