No problem. We give you free $2 to trade with once you sign up. You can use those $2 to open a position with a size between $2 and $300 (depending on the leverage you choose to use).
Should you later desire to open a bigger position, you will need to exchange FIAT money (USD, EUR, JPY) for cryptocurrencies.
You can do so at Coinbase, Bitstamp and many other fiat-to-cryptocurrency platforms.