The best place to start is with the Google AdWords keyword tool. This took is easy to use and can help you find keywords that get enough traffic but aren't all that competitive. If you're not sure of the competitiveness of a keyword, look to see how many people are advertising for that keyword (using the tool) and using supplemental tools as well.
Once you have identified the right keywords for your web site, focus on search engine optimization best practices and PPC marketing. By using these two techniques together, you can start to deliver quality traffic that gets results.