To speculate that bitcoin's price will fall, you must open a forex brokerage account with a broker that offers bitcoin CFDs, or contracts for difference. Using a CFD, you can open a sell order (bet the price will go down) and then place your trade. To make a profit, the price of bitcoin must fall. If bitcoin rises in price, you will lose money. If CFDs are not available, using a futures or options contract can be an alternative way to bet against bitcoin. However, bitcoin futures and options are very complex instruments, not widely available, and should only be traded by professionals.