You'll probably also want to take a look at the website for the US Consulate in London Visa Services U.S. Embassy London for their procedures on securing a visa.

All visas these days require an interview at the London Consulate. Depending on where you're located in Britain, it can involve an overnight stay.

As Rachel said, you will need to show "proof" that you will return to the UK - usually in the form of family or financial ties - and that you have the funds available to support yourself while in the US without working.
Cheers,
Bev