Buy high, sell low.
That lesson was a bit expensive, if you aren't going to study and watch the market for quite some time, don't try to day trade, just HODL, otherwise you'll get REKT.
High and Low are relative terms. Yesterday i sold "high" compared to that morning. Its as if you sell bitcoin when it hits $2,800, and you think you sold high, but over the next few days goes to $3,500.