This is basically called 'short selling' and is part of the reason the stock market went so badly wrong recently. Quite a few fund managers lost a lot of money 'cos they were doing this with Volkswagen shares not knowing that Porsche had been quietly buying up a big stake in Volkswagen. As soon as the news came out the share price went up rather than the down they were expecting and the bugger$ short selling lost loads! Ha ha!
I must admit that I don't know much about the stock market but, from the questions you're asking it seems like you don't know too much either. The stock market is sooo volatile at the moment I don't reckon it's a good time to have anything to do with it unless you really know what you're doing and can keep a very close eye on it - oh yeah, and be prepared to lose what you invest if you don't get it right.
With regards to brokers there's loads of online ones for small sales but make sure you know their charges and most wont offer any advice for small trades so it's up to you to do your homework on whatever you're buying. If in doubt though go with someone you've heard of - I think Barclays have a stockbroker section, for example.