This is just one method to start keyword research.
Adwords has a nice feature, it will analyze any website and tell you what Google thinks that site is targeting.
Open a Google Adwords account, they're free.
Analyze a competitor's website in your niche.
It will provide you with :
A list of keywords that site is competing for.
How much traffic a particular word or phrase is likely to bring to your site.
It will tell you how competitive the word or phrase is.
And much much more.
When you learn how to use Adwords research features you will find it is not just for Adwords.