If you have an adwords account, it’s always better to log in and use the keywords tools from inside your account, but if you don’t, you can always use the Google AdWords: External Keyword Tool.
Enter your keywords and the letters in the validation box, and the External Keyword Tool will show you the top 100 keywords, but no more.
However, a neat little trick is to sort the page by each column, ascending and descending, and that way you will see a wider sample of the keywords returned even without an AdWords account.
You can use the keyword tool to find sub topics in your areas of interest, handy for isolating a niche with high interest and low competition. To do that, just click on the competition column to see first the high competition keywords (you don’t want to focus too much on them) and then on the low competition keywords. Any keywords that show up on this screen with reasonably high search volume (30,000 per month or more) are your golden apples.