You can Register a domain name for less than $10, if you can find one that you want.

For beginners, a good first step is to go to a site like Godaddy.com or NameCheap.com. There you can enter the domain you want to get, and it will tell you if it's available. If it is, you can register it with PayPal or your credit card. If not, Godaddy will suggest some similar domains.