100 usd is more than enough to start trading, you will earn profit based on your capital so if you start with smaller amount then you will need more time to trade than someone that use 1.000 usd. At the end of the day you should use the money that you can afford to lose and don't force yourself to put a lot of money. You can use that amount to trade and if you think you are doing good then you can put more money later.