VTI (Vanguard Total Stock Market) + BND (Vanguard Total Bond Market).
If you're saving for retirement, 30% BND + 70% VTI is a good starting point. Bonds grow slower than stocks, but stocks are riskier than bonds. Both grow over time.
VTI charges a 0.03% fee/year.
BND charges a 0.03% fee/year.
These are very low fees. The management style is hands-off (which is why its so low): VTI buys every stock in the stock market proportional to their size. BND buys every bond in the bond market proportional to their size (ie: mostly US Treasuries, but also some company-debts). Since these are broad and diversified, you should perform decidedly "average", which is fine.
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If you're saving for something near term (ex: new car, new house) that's within 5 years, you'll want to be more-bonds and fewer-stocks, 50/50 or maybe even 70% bonds / 30% stocks
Research bonds and stocks very closely. Learn their details, how companies work, dividends / profits are distributed (in particular, learn the theory between dividends vs capital expenditures vs stock buybacks).
For Bonds, learn about inflation risk, interest-rate risk, and more.
Once you understand the basics, feel free to branch out and put small amounts of money into specific stocks (or specific stock-sectors).