Mutual funds and ETFs are both types of investments that represent a group of individual stocks and are generally managed in some way, either by a person or by a fixed algorithm. Mutual funds have some tax implications that can by annoying for people so ETFs tend to be preferred for taxable accounts (in the US at least).

1. Open a Vanguard account.
2. Buy as much of the thing called VOO as you can each month.
3. Come back at retirement age to oodles of money.

Exchange Traded Funds, basically these funds pool money from a lot of small investors and give you diversified portfolio. Basically if you try to mirror S&P 500 yourself, it'll require about a few hundred thousand dollars to manage, but by pooling with other people you can have S&P 500 level diversification for 100$