You first have to find an exchange that allows users to short sell. In crypto there's a few, but I believe BitMix and Bitfinex are the 2 largest.
Short selling is where an investor sells an asset (in this case, Bitcoin) at current market value by borrowing the asset to sell from an exchange. At a later date the investor must buy back the asset at current market price.
In other words, when you take a short position, you're betting the asset will drop in value. If it does, you'll be buying it back at a much lower price than you sold it for...and keeping the difference. Make since?