Most of the replies you will get here will be to buy bitcoin and not eth because altcoins are risky. Bitcoin is also a volatile asset, there is no guarantee your investment will double. Invest with the mindset that bitcoin is not a get rich quick scheme. Invest only what you can afford to lose. Should your $100 eth or Btc reduce in value will you be able to bear the loss?
Very good advice. $100 to some people is not a big money, but to some people it's like $1000. People are advising him to invest in bitcoin and wait. For how long will he be able to wait. Even if bitcoin moves from $21k to $42k, he can only be able to make $200 which is a very bad business, depending on how long it will take.
Honestly, if Op has some trading skills or if he can learn. Day trading altcoins with $100 will be a better option for him. He could get 1 to 5 dollars daily to add to his capital. By the end of 30days, he could double his capital if he removed greed.
I do subscribe to this your advice. I will put it into practice and hopefully I will come back here to testify.