Quote:My next problem is when I select a part of my image I can't work in that selection. I want to change size and color, mirror it or move it like once whas so easy in my unfortunately lost Photoshop.
and
How you make a new layer of that selection ? I tried but can not.
Gimp is not Photoshop, do not assume the same workflow.
A selection is a just boundary for the enclosed area. You can change the color of the canvas with a variety of tools, also the GEGL tools will work inside the selection... just experiment.
...but it is just a boundary, Gimp does not automatically capture the enclosed pixels.
First copy them. Edit -> Copy .
Then several ways to make a new layer from that, just look in Edit -> Paste-As (new layer * new-layer-in place * new image)
Then remember to turn the selection off Select -> None