There is no limit to start trading, you can even start with 10 USD! But I would recommend starting trading with at least 30-50 USD. As the bear market runs, 30 USD is a good amount in the ETH Calculation. You can start trading from 10 USD to Unlimited in any exchanges except Idex. Because to trade in idex you must need 0.15 ETH. To learn to trade, I think Bibox and Kucoin are better and hotbit and UEX Exchanges are also doing well.