It depends on one's circumstances, but there are still some universal truths when it comes to good investing.
A notable one: You shouldn't invest in individual stocks/bonds. Why? Because if you choose to put your money into individual stocks instead of a well-established index fund, you're essentially betting that you know how to choose stocks better than the world's largest investment firms, which are armed with professionals whose entire lives revolve around making optimal investments. One investment expert (John Bogle, I think?) analogizes it to playing a game of tennis against an unknown opponent who's probably a professional tennis player, and betting that you'll win. And it's possible that you'll win, but it's a poor bet.
Now, there are numerous mutual/index funds out there that will skim as much money as possible from your savings in the form of fees and non-fiduciary investment choices. These are scams. Unfortunately, they're everywhere, from television ads to even some 401(k) plans. But fortunately, it's easy to ignore them in favor of a good index fund. There are just a couple of very basic metrics that tell you whether a fund will be favorable to you. If you're given a list of options under your 401(k) plan, for example, you should—under U.S. law—be given a statement comparing them, and you need only look through the statement to find the plan with the lowest expense ratio and highest rate of return. In the U.S., there's a decent chance it will be a Vanguard plan, if offered.
The only caveat I can think of to simply choosing an index fund with the lowest expense ratio and highest rate of return is that it will probably be a fund that's largely made up of stocks, which are considered risky. But if you're a 20-something who's decades away from retirement, this is fine. Even if the stock market were to crash tomorrow, it's certainly going to recover within a couple decades, and thus will be more lucrative than bonds during this time. I would even argue it's worthwhile for a young person to invest in a fund that's 100% stocks.
Of course, not everyone is able to invest in these funds in general, because they may not have enough money leftover after paying for basic living expenses. In this case, property is often the only viable investment, because the monthly cost of rent may roughly equal the monthly cost of a 30-year mortgage. Indeed, this is the only way many people are able to accrue wealth in their lives.