Brainstorming ideas of one person in your niche, the problems that person has, and what he/she would likely search for online would be good ways to start coming up with ideas. Once you’ve done that, you should be able to identify what to search for on Google.
For example, for self help – there are many people who are interested in self-help. If your niche is married couples (just choosing something out of the air), and you think of the problems married couples might have – disagreements, money, in-law issues, porn addiction – to name a few, then you can start looking for fora that deal with these things. I just did a quick search and there are porn addiction fora as well as broad-based marriage fora.
If you have a keyword tool, that may be helpful as well. But I think your first step is to figure out who your ideal customer would be and go from there.
Hope that helps.