First, bravo on your English! It's clear enough, don't worry. I don't have a lot of experience with Layers and Sheets, so no help there.

But selecting objects is pretty easy. Though I've had exactly the problems you've had. It just takes getting used to.

First, try zooming WAY in, until you can visually identify the objects you want to select, as distinct from those you don't want to select.

Second, the select interface allows you to either click on the object, or create a selection box. Sometimes it's easier to do it with the selection box.

If you accidentally select something you don't want, you can shift-select and it will be subtracted from the selected set. Shift-select will also add something to the selected set, it's like a toggle.