You can short Bitcoin (with respect to USD or other cryptos, but I assume you're interested in shorting BTCUSD) on pretty much all the bigger exchanges. Bitfinex offers it, Kraken too. You'll have to pay a lending fee for the BTC that you short on Bitfinex, which differs depending on the current short-to-long ratio (more people shorting = higher lending fees) and which can get quite high in extreme cases, so long-term shorting on Bitfinex can become an expensive endeavour. Kraken on the other hand has a fixed fee for open short positions, which is quite affordable also in the long term, but they have a limit on the amount of BTC shorts that they allow to exist concurrently on their platform, so it's possible that you can't open any more shorts until someone else closes his positions.

Shorting can be quite lucrative, especially during bear phases. However, you can just as easily burn your fingers - I did quickly earn a nice 4-figure amount shorting ETH (and some BTC) in the middle of the year, during the first bear phase, but I got caught up during the bull phases afterwards, since I expected a pattern more closer to the bursting of the 2013 crypto bubble which was an overall bear, but with bull phases in between. This time it was an overall bull phase with short bear phases in between, and by the time I accepted my fail, I had lost all my nice shorting proceeds (and quite a bit more, but made up for that easily with price appreciations of cold storage cryptos, so I'm at least net positive now).

Shorting is king when the overall pattern is a bearish one, but when it's not clearly bearish, you've been better off longing at good entry points, as cryptos have historically been more likely to grow in price than to shrink. But as always...past market trends are no useful indicator for future trends.