1st you need to visit the US embassy or consulate closest to you and get all the requisite information.
It won_t be easy to LEGALLY immigrate.
Depending on where you are from and your circumstance look for 2 to 7 years to get a green card (makes you legal to get work), than another 5 to 10 years after that for citizenship.
If you have a special skill you may be able to work sooner with an H1-B or other type of VISA, but you need to be sponsored by an employer for one of them (basically have a job already before you come to the US).
Check with US companies with offices in the UK and UK companies that operate in the USA to apply for jobs. Big hotel chains like Hilton and Marriott hire international staff. Most government agencies in the USA have a citizenship requirement, but an exception is the City of New Orleans:
http://www.cityofno.com/
http://www.cityofno.com/Portals/Portal50��
An option you might want to consider is going to university in the USA. Obtaining a student visa is much easier than getting a work visa, and you MAY be able to work on-campus while attending university. That will also give you a diploma that is more easily recognized by US employers, and the university will help you search for jobs as you get close to graduation. Going to school in the USA long-term will also help you decide if emigrating is really what you want to do.
As examples, some universities where I live with many international students are:
http://www.loyno.edu/
http://www.tulane.edu/
http://www.uno.edu/
UK citizens can visit the US without a visa under the Visa Waiver Program, which makes traveling for interviews much easier. You can_t work or go to school under the VWP, but job/school interviews, tests, and medical exams are OK.
Good luck!