Most funds have a minimum dollar investment. I believe Vanguards s&p500 index - VFINX is $3,000.
Anything less than the minimum, you're simply not worth the company's trouble.
Sorry one hundred bucks won't do it.
Are you in the U.S. Web search "discount broker". Most have no fee to open an account. However, you do need to add money to the account, so you have something to invest with. There websites should say the minimum deposit needed for that.
What are some investment companies that have a 100 dollar fee or below to open up a s&p 500 account for saving money