To sell "short" a stock you must first borrow the shares from someone who has them, typically a broker, sell the borrowed shares, assuming that the price will go down, buy same shares later at a much lower price, return them to the agent you borrowed them from, plus paying some small interest, then count your shekels.

When you trade futures contracts you do not need too physically own the commodity, say corn, in order to sell the contract. One sell order cancels one buy order and vice versa. You must exit the trade with zero balance of buy/sell orders and hopefully a profit.