If you're reading a tutorial or something online about some specific steps to do in the terminal, and you don't understand what each step is doing, don't run that step.
Make sure you understand what you're typing in, and what it's supposed to do before pressing enter. This will save you from most problems. Come here and ask if you want to know what each step does and you're not able to figure it out on your own.
Also, if your files are important to you, make sure everything is backed up.