I've been there and done that...my business started in my home office and is now quite successful.
First, stay realistic about your idea. Is it something that competes with companies that already have a long-established market share or a network effect that you cannot overcome? Don't build a word processor and expect anyone to buy it. However you should probably avoid any market that has no competitors. If no one has succeeded in the market, it is probably because there isn't really a market. If you meet both criteria and have faith that you can successfully execute the development - and that people will spend cold, hard cash to use it or advertise on it - then go to step 2.
Second, you really do have to build at least the kernel of the product - the more the better as long as you take no more than a few months to get it together. You simply must have something you can show friends and family and ask "what do you think?" If you are simply thinking (dreaming) about creating something, you'll have no foundation for getting real, usable feedback. Assuming you've created it and your friends have seen it, or better yet have come to use it, then you're ready to move forward.
Third, you'll need at least a few thousand in cash to get started. Savings, a loan from friends or family, whatever. This is, first and foremost, because you'll need to get a website up, a Google AdWords account, and possibly some magazine ads in trade mags. I knew that my business was going somewhere by attending a trade show. It was on unsteady legs at the time (the product that is) but, when I saw the response at the trade show (we sold 30 copies on the spot) I knew I had a winner.
Fourth, it really helps to have a supportive spouse. When I got started, my wife did yeoman's work helping me answer phones and inquiries while I continued toiling at my day job (she was home with our second baby). Of course, even if your spouse doesn't help, it helps if he or she is supportive!
Finally, if you are really breaking out, you may want to invite in people with money to help you move to the big times. If it is a niche product, you probably want to avoid this (as I have). I've seen two companies started by dynamic, knowledgeable people get yanked out from under the founders by investors who grew impatient.
Throughout the process, figure on a rollercoaster ride and some incredibly long and hard hours. Oh, and one more thing, don't bother with Health Promotion or Fitness markets - I've already got 'em covered ;0).