Investing $100 in Bitcoin is a good place to start, at today's price of $21802/BTC will give you 0.0046 btc. The way I see it, don't focus on thre dollar value but rather be focused on the amount of bitcoins you have stacked. You get a good feeling when you hodl a good amount of bitcoins. Buying today is a good investment because you would be buying dip should Bitcoin price fall down further you can still buy more and hodl
I also started with $100. In this field, every day is a starting point, so don't be discouraged. As long as you continue to learn how to strengthen your beliefs in the market changes and don't be greedy for money, you are the winner.