Public companies, those that trade on U.S. stock exchanges, are considerably easier to research than privately-held companies because of the requirements places on them by regulatory bodies. Much of the information you want to access will be readily available on the company's website (see Investor Relations), various Internet sites, or accessible through a number of databases, so check a library close to you.
For U.S. public companies, If you are looking for information contained in SEC filings, the Investor Relations area of a company's website will often have the most current few years of filings, conference call information, and other information that may be of interest to investors. This type of information and discussion is important in researching public companies. Since most researchers are concerned with looking for current filings/information, see the Financial Information tab of this guide for information on how to look for that information. How a company's website is organized may provide a sense of how they do business. If the company has an About Us area, that may be where they provide a company overview, history information, etc.
Also try directory sources which may provide details that just aren't found on a company website like competitors, sales/employee figures, etc. While there are some still in print, databases like Mergent Intellect and Data Axle Reference Solutions (previously ReferenceUSA). They are good for information on a company or companies but you are looking at who their competitors might be these databases can allowing someone to tailor their search and their output. Users to limit their search to a specific geography (city, state, metro area, etc), industry, company type, etc. and may even have the ability to choose what data elements they want to download.
One thing to note is that the Securities and Exchange Commission (SEC) did not always require companies to file electronically so the SEC's EDGAR system has filings from 1994 though there are some from 1993. Prior to that it will be necessary to consult paper copies or subscription based services from Thomson Reuters and eventually the Mergent Archives historical SEC fillings module. Lastly, reports published by investment houses on particular companies are expensive and not readily available.