In general, to short a stock, you need to be able to borrow shares of the stock to sell. For that, you'll need a margin account with a broker such as ETrade, TD Ameritrade, et al. (Not endorsing any of them, just listing names that come to mind.)

Let's take a step back and discuss what you're doing, though. Shorting a stock is a way of betting that it will go down, but it's not the only or necessarily the best way. Remember, taking a short position in a stock has potentially unlimited downside, depending on your margin arrangement: the stock price can always go up, and if your margin gets called, you could end up losing a lot of money.

Two other relatively easy ways to bet against a stock are to buy-to-open put options (limited downside) or sell-to-open call options (unlimited downside).

A put option is the right to sell a particular stock at a specific price before the expiration date of the contract. So, if you own a put option, you can sell stock for some amount, no matter what its present market price is. So, if the price of a share is very high right now, and you think it will go down, you can buy puts, wait for the price to go down, and either exercise or (more commonly) turn around and sell the puts. The maximum amount of money you can lose is your initial investment: the value of the puts can go to $0, but no lower. On the other hand, you're highly leveraged: if you buy barely out-of-the-money puts on a stock for $1, then the price of the stock drops by $10, your puts will potentially go up in value by $10 or more (the price of options is strongly influenced by the volatility of a stock and by its recent history), so you could make 10x your money.

If you sell call contracts, you're selling someone else the right to buy stock for a given price---and agreeing to be the counter party if the options get exercised. If you think that a stock is going to go down, but everyone else thinks it will go up, you can sell call options contracts. Then, if you prove to be right, you pocket the money you made selling the contracts and you're done. Of course, if the stock goes up a whole bunch and you don't have shares to cover it, you could be forced to buy shares at a much higher price than the contract, and you'll lose money. You can see how in this case the downside is unlimited.

So: shorting gives unlimited downside and leveraged upside (you pay someone interest to borrow shares that you sell and then later buy back and repay when the stock has gone down). Buying puts gives limited downside and leveraged upside. Selling calls gives unlimited downside and unleveraged upside (you make what you sell the contracts for, and nothing more).

(These are rough approximations to the truth, and if you're going to engage in short selling or writing naked calls, you'd better learn a hell of a lot more than reading one post on the internet from some random guy who can't even spell his username correctly.)

Buying puts is a far, far safer way of betting against a company, and you don't need a margin account to do it. If I were you, I'd strongly consider doing this.

(I regularly buy puts and calls as a way of making leveraged short-term trades. In fact, pretty much all my short term trading is options, and my long positions tend to be longer term. I do not regularly short or sell uncovered calls, however, because these are frankly dangerous pastimes.)