What should i do with a bull under my arm well i is a big bull under my arm and it has not develop a head

Abscess/node: It seems likely you are describing either an abscess or a swollen lymph node. Either way you should see a physician, probably a surgeon, who may put a needle into it to drain it (if an abscess) or may cut it open to allow drainage, or may biopsy it as a possible tumor. Without knowing your age, how long it has been there, etc it could be a lot of things. See a doctor promptly.