Info for a beginner in SketchUp

Hi!
thanks for your answer!!!
i have just edit and uploaded my skp using your suggest! Many thanks!
For layers, ok, maybe I could understand that is a tag for visibility… I use VERY VERY FEW photoshop and I have the same idea of layer (ok it is a 2D way, not 3D way).
Thanks for all your advice but I have some question!
I number my question with the number of your suggest.
advice 1: why? In this way I can not set not visible this geometry!
advice 2: I set the active layer the layer where my drawing geometry will be put! eg: if I extrude a wall in my second floor I set active layer the layer of seond floor!
advice 3: Ok, but why not use layer for this purpose? I not know what is it a group, what I know is that if I select more geometry I can place them in a group but I now know how manage it… I will document myself about it…
advice 4: I suppose is that I’m doing, for me the second floor is ALL gemoetry of second floor (yes, you are true, maybe I must to use more layer, or group, for second floor, not ALL in one layer… I can work on it). I use camera view from front and select all upper the first floor… this for avoid select every thing separatly (or, contrary to your advice 2, I select active layer the layer where my gemoetry will be put).

Thanks,
Massimiliano