Domain Name = Website Name
The best domain name for your site is to name it the same as your website name or your business name. For example, my site is called Simple Strategy Online, so I choose domain name SimpleStrategyOnline.com. Other example, if you sell photo frame called Shikaku, so your best domain name would be shikaku.com.

Using Keywords
Keywords on domain names are important to help you get high ranking on the search engines. For my previous example you can use domain name such as: shikakuphotoframe.com OR cheapphotoframe.com. Shikaku.com will still the best to use, but then again, you also want to think a strategy to get your site listed high on the search engine. Thus I suggest you get 2 domain names, shikaku.com and cheapphotoframe.com that both point to your website. SO, one domain that is your website / business name that people will remember it easily, and one domain that is specially used for search engine promotion.

Long or Short?
You can have up to 67 characters for a domain name. It is really up to you, for easy to remember domain name, sure you need a shorter domain name, but for search engine promotion you can use long domain name.

Hyphenated or Not?
You can have hyphenated domain names, example: cheap-photo-frame.com. The disadvantages using this kind of domain names are that people don't remember it easily and seldom forget the hyphen. But the search engine will be easily distinguish your keywords better. SO, this type of domain name is good for your keywords domain name that will be used for search engine promotion.

.com .net .org or?
There are many domain name extention available right now. .com is the most common one, you should go for .com first. .org is usually used for a non-profit organization. Most of the time, people use other than .com is just because the .com is not available anymore.

Alternative
Most likely, the short domain you want is not available anymore. If shikaku.com is not available, you can use: myshikaku.com, shikakuonline.com or theshikaku.com. If you insist to have the domain name you want which is currently unavailable, you can always wait for it be available for you to buy, amazingly there are thousands of good domain names expired daily that you can buy. There is a tool that will tell you such domain names daily, you can click here to learn more.

Conclusion
If you only have money for one domain, you should go for the short domain that describe your website name / business. Whenever you're ready to spend more money to get additional domain name, then you should use the long domain name with keywords on it.