Your domain name is the first thing a site visitor will notice. A good name will speak volumes about your site even if someone has not yet visited it. You can easily register your domain by visiting any domain registrar’s site. Before actually registering the domain you need to check its availability on the Internet. Once you find the domain of your choice can also look to its variants. So for example if you have registered, mybooks.com, it would be a good idea to also register mybooks.org or mybooks.net. This will help you to get more traffic by setting the other two domains as pointers to the main one.
Should you ever need to re-brand your company and change its domain name, you simply would need to login in to your domain account and change the nameservers of the new domain so that it matches that of your old one. Similarly you need to remove the nameservers of your old domain. Usually new domains take upto 24 hours to fully start working in the browser as this is the time needed to propogate itself through the various Domain Nameservers across the globe.