How do I invest in S&P500?

Ok, I read this article and it quote: "Melanie decides that pive investing is more her style, so her investment vehicle of choice is the S&P 500 index fund. This is a mutual fund that is indexed to the S&P 500, which is composed of the 500 largest companies in the U.S. " ...How do I do this? Do I have to buy 1 stock of the top 500 companies at a time or could I buy 1 stock that consist all of these companies? If I could buy the stock that consist all of these top 500 companies, what is the symbol? I am new to investing and I would like to start investing but I want to get my plan out first. Any comments would be appreciated.