If you really haven't a clue, start typing in words using the Google keyword tool:
You want topics with a large number of searches and LOW competition.
You can gauge the popularity of certain topics and subjects. The more popular the more traffic you will attract, and more traffic will earn you more advertising income. Basically, write a lot of good quality hubs in topics that others are looking for.
Good quality takes time so try to set yourself a schedule - say a hub a day or a hub a week. With practice your writing, traffic, and income will improve, but it won't be overnight! Experts say until you reach the 50 hub mark, you won't be making much money. There are plenty of Hubs about this, so read and try to follow the advice who have traveled the same road. Good luck