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.
If it offer 100x leverage, and you are in the wrong site, you can easily lose all your money. It is better not to use them.