Risk

Every investment decision is really two decisions stacked together: the upside you're hoping for, and the downside you're accepting to get it. Most beginners spend all their attention on the first one and almost none on the second.

Risk tolerance isn't a personality trait you either have or don't — it's a practical question about your own situation. Money you'll need in a year behaves very differently, in terms of what you can risk, than money you won't touch for a decade.

Diversification is the simplest risk tool there is: not putting everything into one company, one sector, or one bet, so that a single bad outcome doesn't take the whole portfolio down with it. It won't stop you from losing money, but it stops one mistake from becoming the only thing that matters.

Position sizing — how much of your portfolio goes into any single idea — matters just as much as which idea you pick. A great call sized too large can still hurt you badly if it's wrong; a mediocre call sized sensibly usually can't.