Part of selling a product is in knowing who you are going to sell it to. The worst thing to do is to have a great product that no one wants. Before you do all of that work, start off defining your niche.
Do Some Research
Yeah, no one wants to hit the books (or in online business - the search engines), but it is necessary if you want to be successful. Just like you would research keywords for your articles, blog posts and other areas of search engine optimization, research is important for narrowing down the best niche for your product.
Where do you start? Some have a product and then look for the niche. Others have decided on a niche that they would like to pursue, but don't have any idea of a product or service to offer. In both cases, see who is interested, if there is any interest at all.
Let's say that you have decided to do something with pets. Well, there are tons of pet species. Maybe you like dogs. Even then, there are still hundreds of dog species that people all around the world have as pets. Since you know that you want to pursue dogs, choose a specific dog. Is there interest in pugs? How about schnauzers? Who will you look at, those who already have that specific type of dog or potential owners of the breed?
That last question is important because of how you market your product. If you have a special dog collar or a pet bed, the target market will be people who already own that particular dog. Offering content on your website about how to get a schnauzer to sleep in their bed won't necessarily interest someone who wants to buy a dog but hasn't decided on a schnauzer.
Join a Niche Forum
Are there a large number of schnauzer owners out there? If so, is your product unique? Find places where schnauzer owners hang out online and investigate. Don't just ask questions like a reporter or an investigator, but get involved to find out what they really want.
Use your research information to fine tune your product or service and your marketing plan for building your list. You don't want your marketing plan to hit too general an audience or the wrong focused audience. Choose a viable niche first to help develop a successful list and increased chance of profit later.