This is super important, really. Many of the people I know live in overdraft or get into debts to banks, to people, etc.
When I was young, my mother taught me one of the most important lessons in life, I think: never spend money you don't have, never get into overdraft, always try to save some on the side for rainy days. I acted by these words, and while I'm not a rich person (yet, yet...), I think that I'm in a much better condition than most people I know. I have zero debts, no mortgage, no loans, and this automatically makes me more peaceful than most of the population, I think :-)