One site I use to trade BTC/USD is bitmex, they offer a 100x leverage which can be very profitable. You can start off with a leverage lower than this. You can start trading with as little at 100,000 satoshi, furthermore, the maker fee is -0.25% meaning you get paid to buy bitcoins.
in margin trading or trading future is very high risk and danger without have skill
in poloniex margin trading with leverage 2.5x much people can margin call
in bitmex use leverage 100x is very high risk, not control lots is easy can margin call and lost all your money