"The ideal is obviously to exit all trades with a profit, but that is not always realistic. Using a protective Stop Loss will help you ensure that losses and risks are limited."
"Yes dad!"

Do not invest more money than you can afford to lose.  Take chances when you are young because you have time to recover and make less riskier, conservative investments as you get older.

my father say don't be greedy and will win

If you're not a day trader or are not playing stocks full-time then only invest with excess money that you have and never your entire fortune, especially if you are not doing your own due diligence but buying based on speculation or hearsay. Cause you'll never know when and how the stock market might crash.
By only investing with your excess money even if the market do crash at least you're not broke, and can at least hold on to the stock and wait for it to go back up. Paper loss is not a loss till you sell it. ...

Diversify your portfolio to reduce risk!
If you invest in things that do not move in the same direction, at the same time or at the same pace, then you will reduce your chances of losing all of your money at the same time or at the same pace. Simple as that.