Starting with $100 is not a bad idea. I have read stories of people who started with an amount lower than that and they are rocking in millions now. My advice is, Don't just jump at any exchange anyhow, without knowing your way around it. Some exchange requires 2FA for security sake and you cannot send coins out without 2FA. Some exchange are easy to understand  though, but you still need to know how to create orders and fulfill orders, also, if you invest on any coin without basic analysis of what the coin stands for. You could run at oss.