How to go about, purchasing a already owned domain name?

Okay well I'm just wondering, what's the process , if I'm interested in purchasing a already registered/owned domain name, I think I got a brilliant business idea, but the domain name, which I won't state specifically by name, but in general the domain name I want seems to be registered/owned, how do I go about inquiring about, like to make an offer to buy it, who do I contact? the site hosting the domain, or the owners, cause they don't seem to be doing much with the domain name, as when you enter the domain name, all you get is a bunch of links to "related searches" & such.