I would start by reading as much as possible. Squidoo, Blogger, Article marketing are all good for learning the correct methods of keyword & niche research.
But in my opinion the real money is made by
a. PPC campaigns OR
b.
SEO methods - Getting your site to the top of search engines for keywords related to your niche.
If you have some cash to spare I would def go the PPC method and try out various niches and set your daily limit to between $5-10 for starters until you start making a few bucks. Then you can assess what is working and what is not and concentrate all your efforts of what you KNOW works.