Please give me an advice to be a successful Bitcoin investor in the future.
A successful bitcoin investor should have basic knowledge of how the ecosystem works. There are many things you need to know about Bitcoin before investing. So my advice to you is to pause your investment plan and invest time in learning about bitcoin. You are fortunate to be in a forum that you can get almost all the information you need about bitcoin.
100 dollar is for personal use. I can use it to anything I want. I use father's credit card for my expenses.
Okay, if that is the case, then it may not be a bad idea to buy BTC with some of that money every month. But there are things you must learn about BTC before buying it. Take note that BTC isn't a get rich quick scheme and you have to understand how to store your BTC in the right way so you don't lose it to scammers. Let's say you store your coins in a centralized exchange or in an online device that you use for your online activities, your funds can be stolen, so things like these is what you must learn about BTC before buying it.
Keeping his BTC in the right type of wallet is very important. There are so many centralized wallets that are advertised as decentralized wallets and OP needs to know about open-sourced and closed-sourced wallets. He should focus only on Bitcoin and shouldn't be deceived by the get-rich-quick promises of altcoins.