i think that $100 is a good amount to start your cryptocurrency trading, it is not small and not big, you can start adding a knowledge to yourself, so you can choose the best altcoin and if can suggest you, you can start with ethereum or another "safe coin" until you get enough experience, then you can play little bit danger like buying out of top 50 or maybe top 100 coin. it will gives you a big chance to get big money whenever it pumps up.