I'm just began playing in the stock
I assume you mean that you're not using real money, but rather you have an account with a stock simulator like the one Investopedia offers. I am hopeful that's the case due to the high level of risk involved in short selling like you're describing. Here is another post about short selling that expands a bit on that point.
To learn much more about the ins and outs of short selling I will point again to Investopedia. I swear I don't work for them, but they do have a great short selling tutorial.
When you short sell a stock you are borrowing the stock from your broker. (The broker typically uses stock held by one or more of his clients to cover the loan.) Since it's basically a loan you pay interest. Of course the longer you hold it the more interest you pay. Also, as Joe mentioned there are scenarios in which you may be forced to buy the stock (at a higher price than you sold it). This tends to happen when the stock price is going against the short sell (i.e. you lose money). Finally, did anyone mention that the potential losses in a short sell are infinite?