For me a must know before trading was always the idea that there is no guarantee of success or profit. I have seen many people whos main knowledge were buy low sell high, and they expected that now their will trade only with profit. I have seen numerous times, how they buy something, add few % to the price, make an order and wait. And they wait for long time. Hours, day, weeks, months, but their order stays unfilled. So a must know is - dont expect that you will be good at trading and get profit with every trade.