At the start of the game, make sure that the game speed is not running too fast. Go into the stock market and buy ٢ million worth of shares in any of the refineries. Once you have purchased the shares the price should immediately rise. Sell the shares as soon as this happens and buy more shares in another refinery. Then, do the same thing again. Once you have plenty of money, buy all of the shares you can before May or June in the first year. Then, every three months you should get enough money from the dividends to either build up a mass amount before building your company or extra cash as you are building your firm.
Sometimes the amount of shares available for the refineries can be very high. If this happens (mainly when you load a saved game), buy as many shares as you can afford. The share price can then go up by thousands. Sell them immediately and keep repeating the process until you get the desired bank balance.