Keywords are very important when it comes to SEO. The more you seem to know, the more likely you are to be found. Write content that contains intelligent content with relevant keywords, and you're on your way to success! But if you don't know how to find those keywords, there are tools to help you. Some of them may seem to do the same thing, but they offer different features or give different answers, so play around with them.
Here are some of those tools:
Free Keyword Suggestion Tools
Keyword Suggestion Tool for Google
Keyword Density Tool
Search Engine Optimizer - Keyword Density Tool
SEO Book Keyword Suggestion Tool