3 ideas
Save at least 15% of salary from your twenties onward and split it roughly equally among three low-cost index funds: a US total stock market fund, an international total stock market fund, and a US total bond market fund. Hold them in tax-sheltered accounts like a 401(k) or IRA. Rebalance about once a year, and shift toward bonds as retirement approaches, so the portfolio needs almost no ongoing decisions.
The biggest threat to a young saver's retirement is not market returns but their own reaction to them. Fear makes them sell after crashes and greed makes them buy after bubbles, so they reliably buy high and sell low. The market penalizes the emotional investor, not the passive one.
Studying past bubbles and crashes, such as the 1929 collapse, the tech bubble, and recurring manias, gives an investor a gut-level sense that severe declines are normal and survivable. Knowledge of market history is therefore a practical defense, not an academic extra.