Finding a provider:
Think about what you need, how much time and money you're willing to spend, and then you'll have an idea where to start looking. Do you want to resell Linux hosting? Windows? Or both? Are you fine with an active forum and a good knowledgebase, or do you really want to use phone support and IM chat? The best reseller host for you depends on how you prefer to work, and what you prefer to offer your clients.

Getting Started:
Lots of people advise market research: find a niche that no one else has exploited, and go for it.
I come at things from a slightly different angle: think about what you really love to do most. Do you like fiddling with the servers? Writing? Teaching people how to build websites? Tweaking software? Figure out what you love, and find some way to channel that passion to the benefit of your business - and that may help you find your niche. At any rate, it will help you know what areas to concentrate upon and to tout as advantages over other hosting companies.

Another thing to realize is that web hosting is a recurring service business. If you're not planning on doing this in five years, you might want to rethink your decision. When people host with you, there's an implied expectation that you'll be around for the long haul - that you're not going to close shop after two years, or sell your customer base after three.

Crucial-Ijan makes some very good points.