How do I actually start investing with just a few hundred dollars?

Start with a low-cost index fund or ETF through a brokerage with no minimum deposit - you can literally buy a single share of something that tracks the whole market. Apps like Fidelity, Vanguard, or Schwab let you start with whatever you have, and the fees are tiny compared to actively managed funds. The key is just getting started and letting compound growth work over time rather than waiting until you have some magic number in your account.