While there is no required amount to start in trading, there is a minimum amount that you can transact set by every exchange site. Like example in Bittrex minimum is 0.00100000 BTC however if you trade with that amount I dont think you will feel the profit that you will earn from it so better increase your capital when you trade.
Yes, we can freely spend unlimited capital to start trading, but most of each exchange site certainly limits the minimum transaction to make a purchase or sale, because each user can be taxed, because the tax itself can benefit the exchange site itself .