How do I start investing in index funds?

Open a brokerage account (Fidelity, Schwab, or Vanguard are all excellent), deposit money, and buy a total market index fund (VTI, FXAIX, or SWTSX). That's it — you now own a slice of 3,000+ companies. Set up automatic monthly investments to dollar-cost average. A target-date fund is even simpler — it automatically adjusts your stock/bond mix as you age.