To make a trade, you need to pair 2 currencies. The trading pairs can be found at the top right corner on the Trade page.
You are able to trade BTC and USD with any other currency on the platform.
If you want to trade ETH, BCH or LTC with one another you must first convert your funds into either BTC or USD and then complete a second trade to your desired currency.
Once you know the pair of currencies you want to trade you can proceed to the next step of executing a trade.