Hi just to give you an idea, it really does start with figuring out your main interests. I did a little brainstorming exercise writing those ideas on paper on a quadrant, something like what I am expert at, what do I simply like to do, what I am interested in general, stuff like that, then develop your niche by eliminating the ones on the list you don't think you are passionate about and limit the list to the top 3 you think you can really develop. Once you have decided you can then try out a google search to see what other people are talking about in that geneal topic to get the idea of the common keywords within the niche or topic. Hope this helos!

I would do keyword research with all of them using the Keyword Tool in your backoffice, or better yet, Jaaxy!

Find the golden nugget keywords in those niches and pursue those first. Whichever one you would enjoy writing about the most and that has good potential, go with that.