What do i need to do to get a temporary visa to work in the UK?

I am an American citizen & i want to travel all over the UK for 6 months. I just want to get a small job at a bar or something for extra money.i am not a national of one of the Commonwealth countries, so how do i do this if i am an American?Would they allow me to do this? What steps do i need to take? Im not a college graduate or student, just an American wanting to travel the UK for my own personal learning experience. Thanks