http://www.disclosure.gov.uk/index.asp?fuseaction=applying
You can find all the info you need here. You have to pay a fee but I can't remember how much it is. Takes between 2 and 11 weeks depending on how many addresses you've lived at and obviously if you've committed any crimes.
I think it's now about �30, keeps on increasing. Even with no crimes comitted and living at 1 address in 20 years, mine took a month or so, so best to send off for one asap.