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

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.