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.
Is he aware of you using his credit card for such expenses? Isn't the $100 allowance meant for such purpose? If you're stealing from your own father, then you got much more serious things to worry about than BTC investment.
(...) but when I see people buying Bitcoin basis on the internet, I feel like I'm too behind myself. 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? I am worried about missing big bull market with halving and etf approval.
"I feel", "I'm worried" - these are emotions, and you should minimise them as far as possible. Making emotional decisions almost always leads to losses.
If the $100/month is all you can afford, then why would you even care if other people invest more or less? What difference does it make to you?
That being said, if you're aiming for a shorter term investment, i.e. buy now and sell at the top of next bull run, then you have to figure out which indicators to use to know when to stop buying and start selling.
And you're unlikely to make any life-changing profits. The times of BTC doing x1,000 or x100 are long gone. The price is unlikely to go up more than x3 or x4. So be realistic in your expectations.