Before you begin company research, you should know a little bit of information about your company. 
Why? It'll impact where you search and the amount of information that you find. Ask:
Private companies are more difficult to research because they are privately owned; they do not have to disclose any information to the public.
Public companies are easiest to research. They are regulated by the government (in the U.S., it's the SEC), sell shares to the general public, and must publicly report company and financial information.
Subsidiary companies will not have their own annual reports, and you may not find information on them in the resources listed on this page. You'll need to determine who the parent company is and research the parent company. Journal articles will be a major source for information on subsidiaries.
International companies also won't be included in some of the databases, such as NetAdvantage. Check Mergent or Business Source Complete for international company information.
Image Source: Dru Bloomfield. CC BY 2.0. Wikimedia Commons.
"Because the 10-K includes a tremendous amount of detail and legal jargon, most companies also produce a more reader-friendly annual report that typically contains product information, photographs, charts, a letter from the CEO, and other company and financial data." Source: IRIN
Links below provide annual reports.
Journal and newspaper articles offer information on companies from an outsider's viewpoint.
Company profiles are an excellent starting point because they provide succinct overviews of a company: a company overview, history, key facts, top executives, major products and services, competitors, and locations/subsidiaries.
Comparative data on company competitors.
Databases below provided extensive financial data.
A 10-K provides information such as company history, organizational structure, executive compensation, equity, subsidiaries, and audited financial statements. Following are resources that'll take you to 10-K's.