Study & know your subject, whether you are gonna sell Golf Balls or Beaver Balls, Bricks of Gold or Bricks of Cement….

Use the various tools like
www.wordtracker.com
www.inventory.overture.com
https://adwords.google.com/select/KeywordToolExternal

and others to know your keywords for that subject.
Try and keep it to a two or three letter keyword name that is descriptive and short.

Remember when you buy a domain, it takes no more time to pick up a nugget of gold than it takes to pick up a pebble. You just don’t come across gold as often.

Choose your domain name wisely, do research, and having a bit of luck is all you need to start off on the right track.