Just my two cents; Like JCShelton, I have two primary sources. One, is research and hard work based on keyword research. The other is research and hard work based on an emotional reaction. Sometimes I get lucky and don't have to research. But most of the time, I still have to make sure my facts as I remember them are correct. Sometimes, time changes things.
Here's two examples: I love toys. Cool, high-tech toys especially. So I find one I like, do a little keyword research, and if the results are good, I research the toy, and write a nice hub about it. I've done well with this approach.
The emotional approach doesn't look so much for making money, as fulfilling an emotional need of mine to write about it. During this Christmas season, I've noticed overwhelmed salespeople being treated poorly by customers. Now mostly, we're all nice people, but sometimes we get wrapped up in our own problems and just don't think. So I wrote a hub about being nice. I can't imagine ever making money from it, but I enjoyed writing it, and the comments it's received.
It really depends on whether you write for pleasure, or income. A great way to begin is to use the Hub pages search box above. Type in things like "Getting started on hubpages", Making money on hubpages", "Choosing a topic".
There's some great resources here!