My uncle requested that “in lieu of flowers, save that same money and invest in index funds”

Hi everyone, I have an odd request.

My uncle recently passed away from cancer. He wrote a section of his own obituary, and in it he said that instead of sending flowers, we should invest that money in index funds. He was an accountant and economics professor and had a wicked sense of humor. He may have been joking with this, but I want to honor his request and actually invest some money in index funds.

Is it possible to get set up and invest with an amount as small as say $100? Any tips on how I go about doing this? I am completely new to investing other than a Roth 401K.