To "Long" a stock is to buy it and hold, a regular trade.
To "Short" is to sell it when you don't actually have it (ie, borrow from your broker) and then buy it (give it back) when its lower in price, keeping the difference.
I'd love to give you instructions on how to do it, but I think you should read the risk involved and how it really works before attempting to do it. Because of your question, I don't think you're ready just yet to do it.
When you invest regularly in a stock, you can only lose 100%. When you invest short a stock, you can lose more than that.
Edit:: Here is a good article:http://beginnersinvest.about.com/cs/.../a/022703a.htm