In the US, a public company: (a) offers stock for sale publicly on a stock exchange, and (b) makes public certain business/financial information as dictated by the Securities & Exchange Commission (SEC).
Private company shares are held by a small group of investors and are not publicly traded. Private company are NOT regulated by the SEC, so they are not compelled to disclose the same information.
Private company research is trickier than public company research.
Adapted from Introduction to the Markets: Public Companies from the U.S. Securities and Exchange Commission.
This database contain profiles on private and public companies. The private company information is limited but may include sales and employment estimates or reported data.
Comb the company website for information. Here are some tips:
Private company information can turn up in articles and might be all that is available. The articles may be about anything from the company's new products and services to financial figures to the opening of a new location. The information available will vary from company to company but can be used to develop an impression of the company's financial condition.
Although private companies are typically not required to file with the U.S. SEC, they need to file at the state level in order to operate. These filings will provide you with basic information about the company and can often be found via the state's Department of State website.
See the example from the Secretary of the Commonwealth of Massachusetts below.
Give the company a call or send them an email. Explain that you are a student and what you are looking for. They might be willing to share their information with you.
Find contact information on the company website, in company profiles or in directories.