Origin

Hammerhead started out as a small two-bit crook. His exact origins are unknown due to his obscure rise into crime. During a criminal gang fight, he was left severely injured. It was the crimial Dr. Jonas Harrow that found him and experimented on him. Harrow reconstructed Hammerhead's shattered skull and made it as hard as steel. Thus, was the birth of Hammerhead.

Upon awakening, Hammerhead had no memories of his past life. All he could remember was being a criminal. He took his inspiration from an old Al Capone movie. He returned to New York and was determined to take over the criminal underworld. He would use any violent means to take down anyone that opposed him.
Recent issues of Spider man have revealed more of his origin. 

“Death of a Wise guy”

Originally the son of a Russian mechanic he lies to a young school girl claiming to Italian (both are school age at this time) and bully interrupts and pummels him. The girl and the bully leave and go see a movie.

Hammer head follows, and when he believes there is no one to witness him, he bashes the couple’s skulls in. Rather than leaving the cinema right away he sits and watches the movie, a gangster film about Al Capone.

Hammerhead was not the only person left in the theater. The murders had been witnessed by a member of the local mafia who recruits him for work.

Next we see the gang torturing his father, who had refused to pay extortion money. Hammerhead steps up and delivers the kill shot after telling his father he was denying him, claiming his father was from Cecily and had died in the war.

 

Powers

With his reinforced steel skull, Hammerhead can "head-butt" any object. He is capable of smashing through walls and other objects. His head can even be used as a shield against superhuman blows.

His weapon of choice is an old style Tommy Gun.