What do you like to write? That is the best place to start, write what you know, write what you like, write what you are interested in. If you're not sure if other people are interested, google it. See what you find.
Always ask yourself "Would this be useful to somebody?" "Would somebody think this was interesting enough to share with a friend?" "Will somebody enjoy this?" "Will somebody learn from this?"
You can see what people like to read based on what they write.
And duffsmom is correct. Hubpages is not a blogging website. Hubpages is best for stand alone articles about a topic. Look through other high ranked hubs (hubs with a score in the 90's) to see what formatting and layout does best.
The learning center has all sorts of great information and I recommend that you try to read through as many of the articles as you can. They answer a lot of common questions. http://hubpages.com/learningcenter/contents