If a domain is already in use, you'll have to buy it from its current owner. The first step in doing this is identifying who that is, by checking the contact page or performing a WHOIS query. Alternatively, you could leave all of the legwork up to the domain broker services offered by domain marketplaces such as sedo.com or dan.com, or see what alternatives are for sale. Should you want a specific domain, domain brokers can be hired (for a fee plus a provision if the sale goes through) who will identify the owner, negotiate the sale, and deliver the Auth-Code to you.