It's good to see you thinking about investing in bitcoin at your young age. It's actually commendable and I will recommend our upcoming youths to start thinking like this because it break the tie of dependent and reliance on another person. However, for the fact that you're still very young, it's important for you to know how bitcoin works before thinking of investing. I hope you are aware that you may lose the whole $100 if care is not taken. I have no idea how well you understand how bitcoin works but I can still tell you that you have long way to go and I will not advice you to invest the whole $100 in bitcoin at this your stage.

It's better to start small for now. When you're starting out, it's a good idea to start with a small amount of money. By doing that, you can get a feel for how investing works without putting all of your money at risk considering how risk bitcoin can be. Starting small also gives you the opportunity to make mistakes and learn from them without losing too much money. With this, if the $100 is on a monthly basis, I would advise you to be investing $30 every month and with time you can increase it.

Note: this is not a financial advice, do your own research as this is my personal opinion.