I started with $300, and at the time it bought me 1 ETH. But since then, I have introduced people to the markets who have bought in when Ethereum was at a price of between $700-1100. I think a minimum of $100 is best, means that you've got money to lose and you can also make some. It's all relative, but yeah, $100 would be my shout to make some decent money. Obviously the more you put in, the more you can make, but consequently the more you stand to lose too.
To start trading, money is the last thing you need, you first learn, and then invest money, otherwise you will lose everything, do not hurry, but learn, there is no other way