While markets go up over time, corrections and even crashes do happen. An index fund, whether in the form of a mutual fund or an exchange-traded fund ETF , is the ultimate «set-and-forget» investment. No matter how you choose to invest, start by paying down your debt and making sure your emergency fund is properly funded. However you approach it, though, make sure you have a liquid emergency fund that can last you at least a few months. While there is potential for big gains, there is also potential for big losses. Another options is a Roth IRA, which allows you to contribute after-tax money.