Three fund types can provide instant diversification
A broad stock market index fund, an international stock index fund, and a high-quality bond index fund can cover hundreds of securities.
The Fidelity 500 Index Fund invests in the 500 largest US companies, mirroring the S&P 500 with a low expense ratio of 0.015%.
The iShares Core MSCI EAFE ETF tracks the MSCI EAFE index for international exposure.