There are many tools available to assist with this, but most prove to be ineffective, difficult to use and costly. When choosing the right keywords, you must choose ones that are both relevant to your site and commonly used.
Relevant visits equal more chances for sales and more chances for return visits.
Now ask someone else who is not familiar with your business to do the same. Now compare all the lists. The words that show up the most often are likely form the basis of your keyword tag for use with the search engines. You should be. You've just developed a keyword tag with is valid for the people looking for the product that you carry in your geographical area.