What are the knowledges that I should have in order to start learning how to handle arduino without destroying it ? I don't know anything about circuits, electronics and etc., but I am a student and I want to start my own arduino projects...
Download the software, install it, install the driver, connect the Arduino board, and try the led blinking example.
The Arduino Uno is the easiest board to start with.
As long as you use the usb connector to power the Arduino board, it doesn't get destroyed very easy.
But once you use motors and 12V, the 12V could easily destroy the Arduino board.
So there is no prerequisite knowledge ?
There is: buy a second board in case you destroy the first one 
Arduino is all about fast prototyping and testing and learning along the way.
So there is no prerequisite knowledge ?
A desire to learn, helps to be logical.
buy a second board in case you destroy the first one
For the UNO, buy several controller chips as you will undoubtedly damage it/them (until you become proficient).
The IDE (Free Arduino development software package) has lots of examples with it.
Go though each and when you understand how each works you will become more competent.
You learn best by doing.
Also look at examples others have done to see how they solved problems.
Look HERE
Search for sites on the internet like this.
https://learn.adafruit.com/category/learn-arduino
Pick up a book or two:
etc.
When you get stuck with something, ask for help.
Thanks a lot !!