To register (buy) a domain name you have to reach a domain registrar. One of the most known and popular domain registrar is GoDaddy.com. The costs for registering a new domain are $11.99 for the first year and $17.99 for renewals/year. My personal advice is to use Dynadot.com to register your domain. It is very user friendly and the costs for registering is about $8.99 and future renewals price is $9.49.