See here:
http://www.usembassy....sa/niv/add_crime.html
If you 'go by the book', you're required to apply for a visa and then (because your application will almost certainly be refused in the first instance) apply for a 'waiver of ineligibility'. The process is complicated, tedious and time-consuming. You have to get a police certificate, which can take up to 6 weeks:
http://www.acpo.police.uk/certificates.asp
You also have to book an interview at the US embassy. (There's usually a waiting list of several weeks). If, as seems likely, your application for a visa is refused you then have to submit an application for consideration by the authorities in Washington. (That takes a minimum of 8 weeks, often far longer). The whole process typically takes 6 to 9 months (although I've seen a post on this site from someone who had to wait over a year, only to have their application refused).
Your only alternative is to 'forget' about your brush with the law and apply for an 'ESTA' (Electronic System for Travel Authorization) as if you've never been in trouble with the police. The US authorities don't have direct access to UK criminal records, so you might get past their immigration controls without any problems. However there can be no guarantees of this.
Chris