How to invest in a company is one of the most basic investment skills to master for those with funds under their management. Perhaps the easiest way for publically traded companies is to purchase their stock via the stock market where they are listed. For this, you will need to open up and fund a stock brokerage account with a broker that handles transactions on the public exchange where the stock is listed.
Furthermore, if the company you wish to invest in is based in a foreign country, you might also need to perform a foreign exchange transaction to sell your domestic currency and purchase the foreign currency in order to invest in that company. An easier alternative for U.S. based stock investors can be to purchase American Depository Receipts or ADRs for the large foreign companies that list their stock in the United States.
Of course, how to invest in a company that is not publically traded is quite another matter. If the company is soon going to make an initial public offering or IPO, then it may be possible for sizeable investors to be included on a subscription list for the offering managed by the investment bank involved. Since IPO stock can often rise substantially shortly after becoming publically traded, this investment strategy might turn out to be quite lucrative for those who can participate.
Some companies are so small or tightly held by the founders that, while they have taken the time to incorporate, they do not yet wish to have their stock become publically traded or listed on a major stock exchange for some reason. To obtain funding, such companies might offer interested investors the opportunity to make private loans or participate in some other form of private investment opportunity that involves profit sharing. Such companies can be approached to see if they might accept new private investors, and they could also be open to accepting financing from venture capital firms.
Source: http://www.businessdictionary.com/article/775/how-to-invest-in-a-company/