Before starting your company research, consider the following:
Most U.S. companies are private: they do not sell shares publicly and therefore are not required by law to report financial information to the Securities and Exchange Commission (SEC). Public companies' filings are available through EDGAR, the SEC's database. Reliable in-depth information about privately-held companies is available via various commercial databases and publications, but it is much more difficult to obtain.
The SEC's website provides information on how to use EDGAR to research public companies: Using EDGAR - Researching Public Companies.
Annual reports: Produced for shareholders, these include a balance sheet (the financial picture at a given moment: assets, cash, IOU's, inventory, property); an income statement (the year's financial events: what profits and losses and from what sources); a statement of change in financial position; and review of new income per share of stock covering a ten-year period.
8-K: Irregular documents signaling "unscheduled material changes" such as name changes or bankruptcies; also include preliminary earnings announcements. How to Read an 8-K.
10-C: The "8-K" for over-the-counter (OTC) companies.
10-K: These annual reports include information about products, services, markets, methods of distribution, estimated cost of research, total sales, and net income for each line of business; the per-share earnings and dividends, and reasons for substantial changes therein; properties, parents and subsidiaries, legal proceedings pending, and more. Reports may be obtained from the SEC. How to Read a 10-K.
10-Q: Although not audited, these quarterly reports update the 10-Ks three times per year.
Proxy Statements: Designed for shareholders who cannot attend annual meetings, the statements may include biographical information about officers, or information concerning any pending issues to be brought to vote.
Registration Statement: These statements provide current information filed in anticipation of issuing stock.