Hello Massimiliano (@holyhope),
Welcome to the SketchUp Community Forum.
After reading through your comments and those of the respondents, it seems that you will benefit from viewing the introductory SketchUp videos.
Two links are provided to get you started.
CLICK FOR SU VIDEO PLAYLIST!!!
It’s best to watch them in order but not mandatory.
CLICK TO SEE THE YOUTUBE VIDEOS!!!
Watch one or as many as you like in a session and always experiment to explore the concepts presented.
When you become more comfortable with SU, feel free to look at and experiment with the more advanced concepts discussed in each of these series: (Links are provided for convenience.)
“Beyond SketchUp”,
the “Toolbar Series” (very important),
“LayOut” if you decide to go Pro, and when you are ready to really step up your skills
“Familiar with SketchUp”
Have fun and Happy SketchUpping!
Just be aware that some minor differences exist between SU Free (being web based) and the other versions of SU that are stored directly on your hard drive. The one difference that leaps out to me is that SU Free can not use extensions at this time.