Start with research.
You need to know Exactly what market your site will be targeting. You need to know what that market is looking for, what you can offer them, and what keywords and phrases your tagrget market are using to search for what you are going to provide.
Once you have all of that, youc should try to select a domain name that uses at least some of the keywords you are going to target.
So, if your site was about, for example, plucking chickens (hey, it's Monday morning, give me a break) you might look to see if pluckingchickens.com was available. Or chickenpluckingtips.com or ... I'll stop now
www.nameboy.com has a good domain name suggestion tool. Just don't buy your domain from them.
-Mark.