The amount of money that you should have in order to start is the such amount, which you should be ready to lose. That's all.
What you said is also good way of starting trading but if you dont want to lose any off your investment and first learn trading then, join bounty and earn free coins and then trade in the exchange in this way you will gain how to trade in the market and even if you make mistake it wont affect you as this coins are free and what ever you are earning is upto your talent. After when you think that you have learned more then you can start investing and trade or better do same way of free coins earning and trading and earn profit from it.