With credit due to Professor Harold Pollack, below is my nine line list of all you need to know to be a successful investor.
If more space were available, I would add a few words to some of the above suggestions. For example, in number 3, if your company offers stock at a discount, take advantage of that opportunity and then sell the stock when possible and move into non-managed ETFs as suggested in number 2.
I cannot over-emphasize number 1. If you are not applying this suggestion, all others are meaningless. Look up The Golden Rule of Investing. There are numerous blog posts on this topic.