Domains are sold on a yearly basis and you must renew them. The average cost per year is $19.99. You can buy your domain for a specific period of time. Whether that be 1, 2, 3, or 5 years, is up to you.
It is completely up to you how involved you are in the creation of your site, but generally the more the better. More involved parties usually end up with a better end product as well as a smaller cost, and in less time.