With access to the Internet, a lot of critical information can be retrieved relatively painlessly. Look at your main competitors and their strategies. This will help you to identify your market also. For example, if you were offering a lawn care service, your end consumer target might be busy households with enough disposable income to pay for such a service. In this case, you would obviously want a high proportion of single family homes, not apartments or condominiums.
Many sources are available to find such areas.
The United States government has a web site featuring census data which provides all demographics at the Zip Code level. Products with mass-appeal can be advertised on the radio, TV, or in market newspapers. Products or services which are targeting to a resident of a specific town might be featured in ads in small town newspapers. Those that are appealing to a very specific audience might be best reached via the US mail. Often, lists of thousands of names can be purchased for a few hundred dollars.