do some brain storming, thhen mke out some keywords, and check its searches in adword and finalize few of them
try to blog walking and googling, some topic like health, finance, or business, see and learn what you find, analyze and take action off course
You should check into niche finding tools if you are looking to sell ads. It will help you find niches which you will be able to compete in and is much better than simply "testing the waters" you should be strategic as search engines are built to work from mathematical algorithm. It is important to use every piece of data to your advantage. Check out Keyword Elite University and Noble Samurai resources my friend.
you can use all the keyword tools but also start thinking about what people in your niche are talking about.
I agree with KB, search some forums about your topic and see what others are looking for information on. Also, write some posts or articles based upon keywords that will bring traffic to your site. This way, the keyword is in your post title. Then build some links based upon those keywords. B