Company Websites
The first place to look for information about a company is its website. Look for sections such as "Investor Relations," "About," or "Annual Reports," which will typically provide company financial information, quarterly and annual reports, stockholder meeting notices, contact information, etc.
In fact, Annual Reports.com is one resource to get you started. Use the website to search by company, industry, sector, etc.
Keep in mind!
1. It is much easier to find information about public companies (companies with publicly traded stock) than for privately-held companies. The law requires much more in the way of disclosure for publicly-traded companies in order to protect the investing public.
2. The larger the company, the easier it is to find information about it.
3. There is generally more information available for U.S. based companies than for non-U.S. companies.
Edgar
EDGAR is the SEC's database that collects filings from publicly-traded companies. Both the federal and state governments play a role in regulating business and provide access to a rich amount of information on companies from 1996 forward. The most important filings of a public company are the 10-K annual reports, 10-Q quarterly reports, annual Proxy Statements (DEF 14A) and 8-K current reports.
Be sure to know the exact name of the company before searching EDGAR.
The EDGAR basic search page.
California Secretary of State
The Business Programs section of the California Secretary of State's Office provides basic information on domestic stock, domestic nonprofit and qualified foreign corporations, limited liability company and limited partnerships. For a fee, the Secretary of State will provides copies of documents filed by a business entity.
The basic business search available through the Secretary of State's website.