Agree that the Adwords tool is probably the best one to start with. If you are doing PPC, make sure you have all of the columns visible in the display - it will show the the max CPC, the monthly search volume and number of competitors for each specific keyword.
Another Google tool that is not widely publicized which is great for brainstorming keywords is the 'Wonder Wheel' - basically you give it a root keyword and it displays graphically several related but different keyword phrases around it in a wheel. You can then click on any of these words to build the list even bigger.
To find it, perform a search in Google - click on 'Show Options' and on left hand side, you'll see 'Wonder Wheel' - click on it and have a play around with it - quite useful for uncovering different keyword variations!