First, check out the domain on whois, and do a complete DNS lookup. There isn't a web site, but is there an email domain in use? If you browse to the site, do you get an error, or is it "parked"? Does the registrant have lots of domains or just a few? Look on sites like sedo.com to get an idea on pricing.
Then, send an email to the admin contact, indicating your interest to purchase the domain and asking if it is for sale, and negotiate. Be patient.
Once you come to terms, I would use escrow.com or sedo.com to actually run the sale. Their process reduces the potential for you paying and the seller not delivering. It costs a few % but is worth it. In the process, you pay escrow.com, the funds are verified and the seller notified. Then the seller "ships" (i.e. transfer the registration) and tells escrow.com and you. You accept and tell escrow.com, and they get the funds. There is a process for resolving disputes. I have used it successfully a couple of times for just this kind of domain transfer.
An alternative is to talk to whoever you plan to host the domain; they may have a service to broker or facilitate transfers.
Finally, sedo.com has a "broker" service which costs $69 on top of the sales/escrow percentage. I have never used it so I can't recommend it.