Choosing a domain name is an important early step in optimizing your site for the search engines. You can only name your site once, so you have to get it right the first time. Here is a tutorial on how to choose a domain name as it meets the following 3 qualifications.
First and foremost and perhaps the most obvious tip in how to choose a domain name is going with something which is catchy and that your visitors will remember even after they have left your site. The shorter the domain name the better in most cases if for no other reason than to make it as simple on your audience as possible for them when manually inputting your domain's URL.
Secondly, your domain name should relate to your niche. A misleading domain name can really annoy your visitors or even make it so that they don't even bother visiting your site because they don't understand what it's about based on its web address. There should be no surprises when it comes to linking your URL's title to its content.
There are exceptions to every rule. Take GoDaddy for example. This is a domain registrar and hosting provider, but you wouldn't know that from their name. They have made a name for themselves through entertaining television advertisements and delivering quality services to their customers for years, so by now, most everyone who is set to register a new domain is familiar with what GoDaddy is all about.
They also focused on the first rule in going with a memorable and short domain name. It may not be related to domain registration, but GoDaddy is easy to remember and even a bit of a silly name which gets stuck in your mind.
Finally, your domain name should be search engine friendly. While experts in the field of search engine optimization are generally split on the importance of including your keyword in your main domain, it's generally accepted that it is beneficial to you to include it if possible.
This doesn't mean that you should go out of your way to squeeze your keyword in your domain name even to the point of where it doesn't look right. Everything else which you do for the most part should be in order to please your site's visitors rather than the search engines, and this is another example of that. Using your keywords in your subdomains, post titles, etc. will more than make up having it in the main URL, so focus on the first two rules if using your keyword isn't ideal.