Bad Words
If you are considering putting some sort of profanity in your domain name, don't: it's a bad idea. Period. This would have never crossed the minds of most site owners, but it does need to be covered.
Putting any sort of profanity in your domain name will only shock people -- and not in a good way! People simply will not come to your Website. Profanity conveys a poor image and even if your site has the greatest content in the world, many people will not even bother stop by because of its name.
Trademarks and Legalities
You should be aware of trademarks while selecting a domain name. If you register a domain name that an established company has trademarked, you may find yourself in legal trouble. You may be forced to give the domain name to the company, and possibly be required to pay them some sort of compensation.
Save yourself the trouble -- do a trademark search and stay away from names that would directly violate the law. If you registered a domain name before a company trademarked that name, you probably won't be in any trouble. However, don't just succumb to threats from disgruntled businesses: do some research and get some legal advice if their claims are serious. And if you do find that you registered a domain name that was previously trademarked, just admit your mistake, rectify it, and move on.
Finding a Registrar
There are literally hundreds and thousands of domain name registrars out there for hundreds of extensions. So which registrar do you want to go with?
Most registrars are probably fine. There are large and heavily promoted registrars such as Network Solutions and Register.com, and then there are large ones among the Webmaster community such as 000Domains.com and RegisterFly.com.
Whoever you choose, make sure they're accredited by ICANN, which includes OpenSRS. Don't be tricked into paying more than $20 a year for a domain name. You don't "get what you pay for" when it comes to domain names. As long as you use a reputable company that's accredited, you should be good to go.
I recommend 000Domains.com to whoever asks me because of their superior support, affordable price and great service. I have been with them for over 18 months and have over 25 domain names with them.
Domain Name Resources
There are many resources out there that can help you find a good domain name. Here are a few of my favorites:
DeletedDomains.com - This is a powerful domain name search engine that will allow you to find recently (or not so recently) deleted domains, as well as on-hold domains. It also allows you to narrow your search to domain names that are not longer than a certain number of characters, or domains that end with a certain word, etc. This is my favorite resource -- I've found some great domains on it!
Whois.net - This site has many tools that allow you to run searches such as the deleted domain name search and they even have their own (although not very powerful) suggest a name feature.
Nameboy - Nameboy allows you to enter a term or two, and specify if you want hyphens or rhyming in your domain name. They will provide a list of available domain names around your selections.
Domain Guru - Domain Guru has a lot of good content that can assist you in the process of selecting a domain name. You will most likely spend several hours at this site reading if you're serious about domain names.
Your Domain Counts!
I'm a firm believer in the fact that the administrator/Webmaster will make the site and not the domain name. If you have a great domain name, but don't work hard or put in the necessary effort, your site won't amount to anything. And if a site has a great administrator/Webmaster, a less-than-great domain name will not usually affect the site's success.
However, it is always better to have a great domain name, than to have a sub-standard name. By registering a poor domain name, you create even more work for yourself. So take some time to find a good one, and once you have something great, register it! You never know when someone else will sneak up behind you and register the domain that you were eyeing. So grab the good ones and hang onto them!