simple example
if 1BTC price $100
you can use $100 buy 20BTC
You also need to understand the risk in doing this. If the price were to fall by 5% then you would have all of your equity wiped out (and the exchange would probably have forced you to close your position well before this happened). Also you would be subject to manipulation by the exchange as it could cause the price to decline and force you to close your position when this is not what the market price is actually doing