There are as many theories on investing as there are investors, so there is no one right response to this question.
Most of the best responses take into account the investor’s tolerance for risk: can you accept the chance that you may lose the entire investment and still sleep at night? Or are you fearful of losing any part of your capital and insistent upon guaranteed returns? In the first place you can roll the dice and double, triple or do even better with your initial stake, and in the second place you will have to accept much smaller – dramatically smaller, but certain – gains. And even those aren’t always as certain as they appear.
Another thing to consider is the time you have available to invest. In your case, I’m guessing that this is “the finish of the school term”, which is decidedly short term. (This is, in fact, no way to “invest”. What you’re talking about in this case, especially if you’re being rewarded for “biggest gain in a short time frame”, is speculation, which is another animal entirely.)
Investing means knowing what you are buying into, and why, and being able to predict with a fair amount of certainty what you expect to get out of the purchase. Because it is “a purchase”, and not “a gamble” if you do it correctly.
Why do you want to buy a part of any of those businesses, and what makes those businesses (in fact, those industries) better than other businesses in other industries? When you can answer the question and refer to facts as you do it, then you’re on the way to becoming an investor.
Furthermore, I would state that “investing with funny money” or even “with other people’s money” is not at all the same thing as doing it with your own money. For first time investors I recommend that you look into “direct investing” and “dividend reinvesting”. Many companies will allow you to purchase less-than-full-share stakes, or in other words, to “micro-invest”. This will help you get your feet wet, to participate in a company’s growth and drive you to do your homework (in a figurative sense; I understand that this is a homework assignment) to learn how markets work in general and how investing works in particular.
Good luck. Enjoy the journey.