I think there are two aspects. First you should ask yourself which topics or hobbies are very interesting and fascinating for you.
Some people focus on technology (3D, smartphones, IT), health issues (diets), sports (fitness), finance (e.g. foreclosures).
Then you should always check whether this topic is rewarding or not. You could check the keywords and the values. A keyword, which is often chosen (more than 100,000 searches in a month), can be very profitable. But you have also to look at the competition (a high competition must be considered as a disadvantage). Another hint for a good keyword is the CPC (cost per click). To find good and inspiring topics it is also advisable to look at bestsellers lists (Amazon), StumbleUpon, blogs about IT and other sites.