Fortunately, you can reap the benefits of the broader market by investing in a low-cost index fund, which buys a diversified mix of stocks for you at next-to-nothing in fees. That's why your New Year's investing resolution should be to start with the aforementioned S&P 500 Index Fund (NYSEMKT: SPY). Why Warren Buffett loves the S&P 500 Index fund, even though he doesn't buy it himself In Vanguard founder Jack Bogle's book "The Little Book of Common Sense Investing," Warren Buffet is quoted as saying, "A low-cost index fund is the most sensible equity investment for the great majority of investors. ... By periodically investing in an index fund, the know-nothing investor can actually out-perform most investment professionals." In fact, Buffett has even advised his estate to invest 90% of his wealth in an S&P 500 index fund after his death.