Keyword determination?

I have an existent site and am about to jump into some SEO exercises. My problem is where should I start in terms of determining SEO relevancy / top keywords for which I should be optimizing? I'm working in a market that essentially has no real competition per se, but the the keywords are heavily dominated by government and edu presences. I need to figure out how to determine what keywords to really go after and I'm not sure where to start. Obviously I want to get the most real words or key phrases as possible so that people actually search for me...