Search engines and directories use keywords to help determine which sites to list in their search results. In optimizing your website for the search engines, you should start by making a list of several keywords that describe your site content. Ask yourself, "What would a person looking for my site enter into a search engine?" Do this for each individual page of your website because each separate page should have specific keywords that are targeted to what that particular page is about.
When making your list, you may want to try a service such as WordTracker to help you decide which keywords to use for each page.
You can take their free trial and do various searches for the keywords that you have chosen to see which get more searches on the search engines.
While your first instinct might tell you to pick the most popular keywords that receive the most searches, keep in mind that there is more competition to rank high in the search engines with these keywords. Look for keywords that are specific to what your page is about, rather than more general keywords.
It is very important to select keywords that are specifically related to each page of your website.
These keywords are going to be used to optimize both your meta tags and your page content.