I would say that the three most important skills are:
- Ability to analyze the effects of various costs, including taxes. This is the most important thing! Under the efficient market hypothesis, there is no other way to obtain better return without increasing risks, than to reduce costs.
- Ability to diversify. So, you shouldn't have most of your money in a single company. Similarly, you should not have most of your money in a single industry. Or in a single country. Thus, if you're in the United States, don't buy just US companies.
- Ability to tolerate risk. There is no easier way to become wealthy other than to bear equity risk. If you choose to bear only partial equity risks (like having 50% stocks), your returns will suffer compared to the approach I recommend, going all in into stocks (perhaps sans a small emergency fund).
Note that some costs are hidden. So, for example, a mutual fund investing in other countries than where you live in may mean the investment target country charges a certain percentage of dividends going to the mutual fund. The mutual fund company doesn't usually want to tell you this. There may be clever financial instruments (derivatives) that can be used to avoid this, but they are not without their problems.
If you diversify into equities at low cost, you will have a very wealthy future. I would recommend you to compare two options:
- Build manually a well-diversified portfolio of 30-40 stocks. Ok, you could have more, but after some point diversification doesn't buy you anything anymore.
- Invest in a low-cost index fund (mutual fund or ETF)
...and pick from these options the cheaper one. If your time has a high value, and you wish to take this value into account, I would say it is almost always far better option to choose an index fund.
Whatever you do, don't pay for active management! It is a mathematical truth that before costs, actively managed investments will yield the same return than indexed investments. However, the costs are higher in active management, so you will have less total return.
Don't believe that good historical return would imply good future return. However, if for some reason you see an index fund that continuously loses to the index more than by the amount of stated costs, beware!