My first foray into the market ended in (minor) tears where I "invest" in a 10x leveraged pair trade (who could have known being 10x leverage to a volatile asset was a bad idea).
Anyway I went away to try to work out what I did wrong (I was actually right on the ultimate direction of the pair, but was so highly leveraged that it was a disaster waiting to happen). I learned about the Kelly Criterion [1, 2] which every investor/gambler should know about. As a rough rule of thumb never invest more than 5% of your capital in any one company or you will lose in the long run even when the odds are in your favour.