It’s hard to pick just one thing, but diversification is huge. The broad market funds (like VTI, QQQ, DIA, SPY, VOO) can eliminate much of one of the two bigger risks investors face. Securities risk or risk that an individual company stock can collapse. Diversification protects against that. The other risk is unavoidable. Market risk. Markets can be volatile. They will fluctuate. It’s part of the reality of investing.
Patience is big. Don’t panic when markets fall. Don’t get too greedy when markets rise. Dollar cost averaging is a great concept. Invest the same amount on a regular basis and market pullbacks are beneficial. Shares get cheaper and the investor will get to buy more of them during those market pullbacks.
Be careful with leverage. Margin can make portfolios grow really fast, but also can send them to zero very quickly.
Discipline.
Define investment goals. Long term investors can take on more risk.
Let winners run. If an investment goes up a lot, don’t sell all of it. Maybe pull out the initial investment amount or a portion of it and let the profits run.
Understand the favorable tax treatments. Roth accounts are one of the best things individuals can participate in. If building wealth to pass on, the stepped up cost basis is another fantastic part of the tax code. Long term capital gains versus short term rates. Etc.
That is more general stuff. But exponential growth and compound growth work better and better over longer periods of time. Compound growth really needs income to work. Like dividends or rent or interest. Crypto, non-income generating real estate, and precious metals don’t really include a compounding element with the math.
There are good points here, as well as what others have added.
In no particular order, here are some of my thoughts. Others always need to decide what's best for them, however.
If I buy a stock or fund that has a dividend/yield, I have it automatically reinvested (called DRIP). That way I automatically get more shares with no transaction costs.
I generally try to keep my expenses to a minimum. That goes for buying stocks or funds. Mutual fund annual expenses can eat away at the return. If I buy and hold a fund for years/decades, there's a big difference in a fund that charges 0.20% a year and 1.20% a year. If I'm looking for an index fund, I look for which company offers it for the lowest expense ratio.
If you're starting out, maybe check out Investing for Dummies or Mutual Funds for Dummies. I don't mean this as a slight to anyone, but it might be a good starting point to learn. Off and on over the years, I've subscribed to the Motley Fool's Stock Advisor. I like to read and learn and they do a good job, IMO.
I talk to people who are smarter than I am and who are professionals (I'm not a professional). I confer with others almost all of the time (except that time I bought shares in the National Bank of Greece- I knew I was taking a chance).
I'm a buy and hold person. I don't use margin, I don't do options, and I don't do crypto (to each their own). For me, boring is good. I think of investing as a crockpot and not a microwave. I don't sell on bad news- in fact, I've sometimes bought. I bought Ford when a share cost about as much as a Matchbox Ford Mustang. Held it since, all dividends reinvested. I've held Apple for almost 15 years. I've had mutual funds for a long time.
Have I lost money at times? Absolutely. I've rode stocks into bankruptcy (further proof I can be wrong). I view investing as a learning process that will never stop for me.
If you pull up a chart of, for example, the DJIA and look at the past 30 years, you'll see how overall it's increased over that time frame, even with all sorts of calamities, wars, and a number of Presidents of both parties. I sure hope that upward trajectory keeps up.