DONT GO NEAR THEM.
As someone who has traded stocks for 30 years, and who teaches stock-trading at a college - I repeat - DONT GO NEAR THEM.
They are attractive because their price is so very low...it makes you feel good to buy 1,000 shares of something with your spare change. But when you add in the commission you'll pay to trade, you still have to spend larger amounts than you think to make anything worthwhile.
But more importantly, penny stocks are constantly the tool of what is called "Pump-and-Dump." Some shark buys a bunch of them, and then starts a whisper campaign that this stock will take off real soon. In fact, he may be buy a shitload at .05, and then buy some more at .07 or .08, just so the 'last trade' makes it look like its rising. Young people like you with only a little money to spend decide, "hell, I'll get on this train!," so you start buying as the price increases to .10....at which point your original shark sells all his shares, and the price falls back to .05 or .04.
Last point: when buying stocks on an established exchange (NYNEX, AMEX, NYSE), buy and sell orders are matched by computer as soon as the order is placed. It is efficient and as fair as can be.
NOT SO with Penny Stocks. Most Penny Stocks are labelled as being "OTCBB" which stands for "Over The Counter - Bulletin Board." That means the orders are not handled electronically - they are literally written on scraps of paper and pinned up on a board. Buy and sell orders are matched manually by whoever walks by next, and they dont have to be matched based on the ordre they came in or on which are the best matches. These stocks can often be identified because they carry the suffix ".PK," which stands for "pink sheets," because thats the color of the paper they are printed on.
Again - DONT GO NEAR THEM. If you want to get into stocks, dont try to day trade and be a hot-shot. Put $100 or $200 into an Ameritrade or E*Trade or ScottTrade account every month, and buy real stocks, and leave them there to grow. If you want to avoid commissions and only have a small amoutn to invest (say, $20-$50/month), contact utility companies and ask for their Direct Investment/Dividend Reinvestment (DRIP) Program. No commissions, flat-rate contributions, fractional shares, and more stiock instead of cash as dividends. Its the best place for a new young investor to start.