The first step in any kind of Company Research is to determine company ownership - is the company public or private? This will usually determine how much and what type of information you'll be able to find on your company.
Public companies, like Wal-Mart, Ford and Coca-Cola, are those that sell stock to investors. They are required to submit information to the Securities & Exchange Commission (SEC), so they typically have the most information available to researchers. Public companies are required to provide detailed financial statements and annual reports to stakeholders. You'll also find a lot of information from news organizations, investment analysts, market research firms, and publishing companies. Public company stock is openly traded, often on one of the large American stock exchanges (NASDAQ, NYSE). So, if you can identify a stock ticker symbol, the company is public.
Private companies, which do not sell stock to investors, are not required to provide information about their companies to anyone. As a result, they can be more difficult to research. If you find financial information, it is usually an estimate. Estimates are sometimes generated based on the type of industry and number of employees. The estimate could be very wrong! If the private company has competitors that are public, you have a better chance of finding some information on the company. Sometimes, all you will find are contact info and what they do.
Most of the databases linked on this page and others in this guide can help determine whether a company is public or private Hoover's is highly recommended for its ease of use and broad coverage. UPDATE: Hoover's will no longer be available as of January 1, 2019. I recommend you start using Mergent Intellect in its place because the source of information is the same for both databases. Mergent provides some increased functionality in the form of downloadable spreadsheets, so there is no material loss in moving from Hoover's to Mergent Intellect, aside from familiarity with the Hoover's interface.
For a publicly-owned company, look for an Annual Report. Produced by the company each year for their stockholders, the annual report will describe the company’s operations, financial situation, and future plans. While similar to the 10-K report required by the SEC, a company's annual report will tend to be packaged with more of a marketing-bent, with language aimed directly at current and/or potential investors. Try these sites to locate a copy of the annual report: