It depends on what extension the domain has. If its anything ending in .uk go to
http://www.nominet.org.uk/ and put the domain (without the www) into the whois search box in the top right. Bear in mind that if the owner is an individual they have the right to remain private and not have their name and address shown on the UK whois.
If it's a .com, .org or .net go to
http://www.networksolutions.com/whois/index.js p. Again bear in mind that owners of these domain extensions can opt for privacy which means another company's details may show up instead of the actual owners.