If you want to select/deselect all/parts use the “A” hot key . You select as you know with RMB .
If you want to use the brush select/deselect hit “B” twice and you should see a dotted brush circle appear . You can increase/decrease the size with the mouse wheel (MMB) . You brush select with the LMB . You deselect by pressing down on the mouse wheel (MMB) .
Now I’m trying to “cut windows” in a solid panel. I’ve tried subdividing the surface then lowering ther selected vertices, didn’t quite work. I may now try extruding the selected vertices downward, the eliminating the faces over the extrusion. The idea is I have a flat, angled hull panel and I want to have some beveled windows in it, vaguely like the ship in the original planet of the apes movie.
In general, I think a better method is simply to extrude->scale in the panel faces, resize the new inner faces to the shape of your window, delete the inner faces, and manually create the “window sills” (interior edges)
alternatively, make two knife cuts down both sides of the panel, directly “cutting out” the shape of your window with the cuts, then delete the inner face and fill in the sills again.
first has less verts but trapezoidal faces, second has more verts, but all cuts are parallel.
Also, avoid booleans like the plague (in most situations, they are almost necessary for creating certain types of objects). At least in blender, the geometry returned by boolean operations is horrendous (I know its gotten better and is being worked on [I think] but still)