There are three rules that are now cliche for finding and buying great real estate, it's "location, location, and location". So here are my three rules for selecting a great domain name, keep it "short, simple, and catchy". Almost all the famous sites have domain names that follow these rules (i.e.: Yahoo, eBay, Amazon, etc.).
The main problem you will run into is finding a great domain name that is available. It seems like all of the good and easy to find ones are already taken. Like finding great real estate, to find a domain name for your site will take time, and research.
What I can do to help you is provide you with some guidance to selecting one.
Tip: Several domain name registrars (such as GoDaddy, and others) offer suggestions for domain names variations if the domain you're looking for is not available.
Rule 1: Keep it Short
Generally the shorter the domain name the better, primarily because in most cases it will be easier for your visitors to remember. As a guideline I would not try to exceed eight characters in length, but remember there are almost always exceptions to suggestions such as this one.
Rule 2: Keep it Simple
Like short domain names, it should be simple to remember, and easy to spell. You don't want your visitors to struggle to figure out how to spell your domain name. Try to keep you names down to one or two words at the most, and avoid names with three or more. A great example of a two word domain name is Facebook.
Rule 3: Make it Catchy
To be catchy these days, you seem to have to have your domain name under the .COM TLD (Top Level Domain). If you're a commercial company, the .ORG and .NET TLDs just don't have the same impact. Although if you truely are a non-profit (for the .ORG) or you are an ISP (.NET) then they may be very appropriate. Other things to consider is the geographical regions you will be servicing, so you might want to register your domain in the local TLDs (such as: .UK, .CN, .JP, etc.).
If you're using foreign words (no matter what language you're targeting) can that have complex pronunciation, or strange combinations of letters, this can be hard your visitors to spell or remember. If you can, try to pick a domain name that is descriptive to what you're marketing. For example, if you're selling time/space widgets, if WIDGETS.COM is not available try WIDGETHUB.COM.
You should also consider if your domain name is brandable. This can be an important consideration to some organizations because their domain can also be their company name.
Other domain name considerations:
Tip: If you still need more help in selecting a name, check out the following article (Domain Name Research Tools).