Shorting

Ok so, I'm trying to figure out exactly how/when/what I can sell short.

After a bit of reading, I gather that you can only short something that someone from your same broker has bought (or something like that). Am I on the right path? I'm using Thinkorswim right now for paper trading and each equity I try to short gives me an error that "the stock is not available to borrow."

Are only certain stocks available in general for shorting? Do only certain brokers do it? Can only somehow-verified accounts do it?

Thanks for the guidance in advance.

To short you need to borrow shares from your broker, if your broker doesn't have them available you could go a long put. You could buy a put with less risk then shorting outright. Although with a put you could lose your entire investment...