How, and why, to invest in a simple portfolio of index funds. Split between equity and fixed income based on your risk tolerance, and diversify equities globally. This will give you a low-cost, set-and-forget investment portfolio that you can add to over time without ever having to worry about what you should buy or what the market might do. Just add to it regularly over time, and end up with solid retirement savings. The market and your portfolio will certainly fluctuate, but there is no need to react to these fluctuations—simply rebalance based on your investment plan (say, annually).
Here's a good primer from the Bogleheads forum: https://www.bogleheads.org/wiki/Bogleheads%C2%AE_investing_s...