There are many keyword tools on the web (SEO Moz, Digital Point, SEO Book, etc). The goal is to find a niche keyword that describes what people would be looking for on your site, but also doesn't have a lot of competition from competitors also targeting the same keyword.
The trick is to find the perfect balance of a keyword phrase that doesn't have a lot of competition, but also gets a significant number of people typing it into the search engines.
If you're aiming for short tail keywords, a handful of really super niche targeted keywords will do, provided they pull a decent number of search queries. Long tail is when you try to rank for thousands or hundreds of thousands of different keyword phrases, but get little traffic from each one, but it all adds up to something substantial. In other words, it's not putting all your eggs in one basket, because if you lose your ranking for some of your keyword phrases, your overall traffic doesn't take a nosedive.