Good keywords are frequently searched for (high demand) but not being targeted by many other websites (low competition). There are 3 tools out there that can help you find them.
The best tool out there, Wordtracker is one of the most essential SEO tools. To use Wordtracker:
Wordtracker also offers a 7-day free trial.
Also very useful, Google AdWords Keyword Tool is free and much quicker to use than Wordtracker. Like Wordtracker, it tells you how many people are searching for each keyword phrase as well as the level of competition.
KeywordSpy is an interesting resource. Type in a keyword phrase and it'll tell you which websites are targeting that phrase. You can then click through to see which other keywords that website is targeting. This tool is excellent for giving you ideas for keyword phrases based around what search terms your competitors are targeting.