When I started trading, I just use 40$. That was my extra money before, and I just try bitcoin because I am really curious on how it works. I started trading november 2017 then luckily on dec 2017, bitcoin hits its ATH, my money got doubled. That was the time I realized that if I just put some more money, I will gain a lot more. And that is what I did which is a very wrong move, I put all my money then the bitcoin price dropped. It was unfortunate but I learn my mistake. You will not be a successful man if you haven't taste the bitterness of losing. So invest what you can only afford to lose.