Well read the guidelines for what is NOT allowed first, but pretty much you can write about anything you want! A good place to start in getting ideas is right here in Answers. Find a question you are interested in and click Make a Hub About it!
And when you write, make sure you use intrigue and perhaps give some advice or facts that the reader can use in their daily life. For an example, read my Hubs "Foods That Fight Stress" and "Slim Down Your Inbox With Twitter." These offer advice and are fun to read. Those are what people are looking for.
Hope this helps!