SPX is the symbol for the Exchange Traded Fund that represents the S&P 500 stock index. But if you just want to invest on a regular basis, you should go to Fidelity.com or Vanguard.com and open an account in their S&P 500 stock index mutual fund. And if you are just starting out and are working, make the account an Individual Retirement Account, so your gains can grow over the years tax-deferred.
And if you really want to know how to get started, I wrote a book just for you:
The Savage Truth on Money. Click on the title and it will take you to Amazon to buy it! I will personally give you a money-back guarantee that you’ll learn everything you know to get started — and avoid costly mistakes!