As the search engine's robots are indexing your web pages they are looking for keywords and phases that will match those typed in by their users. After back-links, keywords are the most important element of search engine optimization. You need to be sure that you include relevant keywords and phases throughout your site. That said, keywords should not come at the expensive of good relevent content. You can sprinkle keywords thoughout the content but be very careful not to create keyword stuffing. Include keywords in your page's structure and labeling which should indicate its purpose and relevancy.
Your Competition
Before you get started check the keywords of your competition. Analyze their site for keywords. You can use the search at Google Rankings to get a page's keywords:
Check the competition's sitemap (if they have one), by sitemap I mean the one that is available to the users not the one that is created for the search engines.
