I'm preparing for the college entrance exam to go to college, so I can't make money from working. I get about $100 of allowance from my father every month. I use all of them to buy bitcoin because I can use father's credit card for expenses such as food, clothes. I can buy Bitcoin on a satoshi basis every month,
It's not a bad idea, but make sure you take care of yourself when investing, don't invest everything you are having, always invest any amount you know you can afford to lose, and make sure you are always having backups and always have extra money that will always serve as emergency money. In case there is any unforeseen circumstance for which you will need money, you won't be selling your bitcoin for that.
but when I see people buying Bitcoin basis on the internet, I feel like I'm too behind myself.
If you want to listen to what is being said by people on the internet, then you will always feel bad. When investing, always invest what you can afford to lose, don't invest what's going to affect you after investing. Don't be surprised that most of the people you think are investing on social media are not holding any bitcoin. Most of those videos you see when they claim they are accumulating a huge amount of bitcoin or they have accumulated a huge amount of bitcoin are fake. So you should be proud of yourself even with the little amount you think you are able to accumulate monthly, and I don't think investing $100 monthly is kind of small or that's why you should be ashamed of yourself. No amount of bitcoin you are holding is too small, just make sure you are holding bitcoin.
Maybe they have a much higher income than me because they have a job. I think I can only invest $100 a month for the next year or two, would that be enough?
All fingers are not equal, and you don't expect all of us to have equal bitcoin. Definitely, there will be some people who will be financially okay to accumulate more bitcoin, and some won't be able to accumulate much bitcoin due to their financial challenges, so just focus on yourself and ignore other people. What you should care about is your own investment and not others.