Keep your purpose in mind throughout the selection process—write it down.
Write the focus and goals of this site, and make certain to answer, "What is my site about?" Make a second list of keywords for your site. Now, coin a list of domain names using those two lists. Have as many options as you can.
Test how available your ideas are at a domain name search engine, and record the results. If dot-com is not available, look for dot-net, dot-org, and other major domain names, and record those results, too. (Note: outside of the United States, you want to seek the primary domain for your country, such as dot-co-dot-uk for the United Kingdom, or dot-fr for France.)
Which domain name will best suit the website's purpose? If this domain name is already taken, consider your budget: might you buy the domain name from the current owner? You can also use suffixes for your domain name like resource, guide, universe, and so forth, but be careful; these can be confused with the other, already-registered domain name.
If you have multiple purposes for your site, you can register more than one domain name for the site, rather than have a long or nonsensical domain name that would hurt your Internet traffic rather than help it.