I'd like to ask about adjective. Which one is the correct one :
a. beautiful young girl
b. young beautiful girl

Is there any rule if we want to put more that 1 adjective ?
Thank's before and after
Both of those are correct. (Not sure about the rules!)
Well, there are some rules to adjective order, but the following is just a guide, not written in stone. This is what I teach my upper intermediate students:

Opinion- what you think
Size - how big?
Most other qualities - anything that isn't one of the other qualities!
Age - how old?
Nationality (origin) - where from?
Material - what it is made from
Use - what it is for

So in your example, following these guides, beautiful (your opinion) comes before young (age).