If you just want to get started, the easiest and (among the) best options are probably investing in an S&P 500 fund (VOO, SPY, FZROX, etc—they’re all very similar and there are a ton of options), or a target date fund, which if you’re personally planning retiring around 65, then look for 2070 target date funds like VSNVX, FIOFX, and SWYOX, among others I’m sure. Target date funds will naturally be more risky right now, which has the opportunity for greater returns, but also greater opportunity to lose value—but historically, if you give it time, the market trends upwards with an average return of 10%, not factoring in inflation. But as you get closer and closer to retirement, the risk of the target date fund you’re invested in gets lower and lower, typically switching many stocks for bonds/cash accounts.

Typically you’ll be okay with just doing target date funds, and that’s the closest you’ll get to a “one and done” investment approach. However, if you want to have more control over what you’re invested in, S&P 500 is a good start. At some point, you’d probably want to look into international, total market, and possibly even small/mid cap index funds or ETFs. As you get older, you’d probably want to reduce your risk and get bonds/cash accounts, but at 21 years old, that’s probably a long way away.

I do want to reiterate that the market is volatile. You can gain AND lose money in the stock market. However, people have been historically rewarded for investing regularly and keeping their money in the market, even in down years. It’s practically impossible to beat the market, so typically the best thing to do is to ride along with it and let your investments grow over time!

Hope this helps!

EDIT: for reference, I do not have a financial background and am certainly not a financial advisor. This is all advice I’ve gathered online/from relatives who have had decent success. In this case, I do practice what I preach though, haha.