You mentioned the wrong person. I'm no aficionado but I presume that the stock market operates on the same rules of supply and demand that the free market is based on. Fewer stocks, and the higher the demand, the higher the price. Vice versa and the opposite holds true. That's why buying drives the price up (higher demand, and decreasing supply), and why selling drives the price down (demand goes down, available supply increases). The price of a stock is usually driven up when a company releases a new product or they have something big cooking on the horizon. People anticipate this, begin to buy stocks, thus driving the price of the stock up, so they can sell it at a later date and make a profit. When they do, other people buy the stock, but the demand goes down. And the people who bought the stock wait for the next spike to drive the stock up so they can sell it.
The stock market isn't a logical free market at all. It's driven by fear and herd mentality. People anticipate others reactions to news and make their moves. Large market movers try to manipulate the market and can generally screw you out of your earnings sometimes, depending on their motives.
Don't ever stick in money that you can't do without in the stock market. You can setup an account under aged, but it would have to be through under your parents generally. Stay away from penny stocks until you actually know what you're doing, and even then stay away from it. It plays by a different set of rules. You don't make "monthly earnings", generally, few companies hand out dividends but they are usually < 1% every 3 months, but you have to factor in the change of value of the company to make a good investment.
To seriously "play" in the stock market, you would need at least $10k, because you don't want all your money invested in one company but it's basically unavoidable after broker fees with low amounts of money, and you want liquid assets when there is a "deal." Once your money is in, it's basically tied down till you sell it.
My advice to you, is to setup a "game" account on one of the many sites, and invest like you would if it's your own money. If you don't screw yourself over or abandon it, you'll know if you can handle the real market. Just remember the real market is a lot more harder than this fake market.
People who say the stock market works after the supply & demand system are... well... not informed.
If that would be the case the Facebook stock would have skyrocketed. It didn't, on the contrary, it went down 40% on the release day.
The stock market was intended to work after the supply/demand system but as dabe said, it is rather luck based.
And also you need to be of legal age to buy stocks. If you interested in buying stocks you need to contact a bank, you can give them the order to buy a certain amount of a stock. You can't actually buy them yourself because they are traded on a corporate level only.
Also when you look at the google page for stocks (Google.com/Finance) you see a $ price.
That is the price of each individual stock.
But usually stocks are not bought in single amounts.
Rather in larger amounts.