As you may know, Waves is the main currency of the whole Wavesplatform. So, before creating an order or outgoing transfer, please make sure that you have enough Waves.
The fee for creating one buy or sell order on the DEX platform is 0.003 Waves. To create any outgoing transfer from the wallet will cost you 0.001 Waves.
If you have enough assets (BTC/ETH/LTC/ZEC/USD/EUR and so on) and if these assets have sufficient value, just exchange them for Waves without having initial Waves using the internal DEX exchange.
For more information please follow the instruction - How to trade tokens on the DEX
Warning When you are exchanging assets for Waves to get the first tokens. Please take into account the amount of exchanging, since order fee is 0.003 Waves. Thus, the amount will be debited from the received Waves after order execution.