There a few things to consider when selecting a domain name to start an affiliate business. You want people to be able to remember it as well as to be able to figure out the topic of the web site when they see it (using words that people might type into search engines to find a site like yours can help).
Make sure your domain name is fairly easy to type. If one of the words you want to use in your domain name is misspelled often, you might also consider buying the misspelled version of the domain name as well and redirecting it to your primary site. If you are using multiple words, consider separating them with a hyphen to make it easier to read.