Alright forget about the z-index i will just give you a straight example how to do this without the need to mess around with the z-index and the action of add another layer .But keep in mind if you are a complete newbie as you say, you have to read the manual to understand the meaning of the z-index and stuff like that.
Here you go.
I didn't want to mess around with overlaping etc. so i just add a simple variable to the wall sprite. (Move the keyboard arrows.)
Edit : i just read that you need this action for isometric 2d style game.So i think zenox98 way its "the way" to do it.