when you try to register a domain, please keep in mind our domain name is always releted to our service.
Try to find domain name which is not in heavy competitive niche. Such domain as e.g www.laptop.com will find it hard way to rank up in Google. If you have easy to access domain, your Adsense will have lots people watching.
long tail domains are fine for targeting specific Kw's. for example of you're trying to target the search term Denver Co Foreclosures you'd be better reg'ing DenverCoForeclosures.com than ColoradoForeclosures.com. shorter doamin names are better for targeting a wider range of terms, but longerdomain names are best for targeting specific queries imho
I think to use some keywords, related to your content for your domain name, if .com is taken you can try .net or .org and eventually .info that is the cheapest Tld and very popular in this period!