Cutting a hole in a "Plane" I decided to design my own room in blender, so made a room out of "Plane"-Objects. Now i need some holes in the "walls" as I want to make windows an doors. 
I have absolut no idea which tool to use to cut something out of a "Plane" a name for a useful function would be enough, anything else should be findable with google.

Btw I'm an absolut beginner so please be precise.
Thank You
Assuming you want rectangular windows and doors:

1) Create a cube and adjust it to the required size of the window.

2) Move the cube so that it goes through the wall where you want the window to be.

3) With the cube still selected, use Ctrl-A, Rotation and Scale.

4) Select the wall object.

5) Go to the Modifiers tab (the one with the wrench on it).

6) Add a Boolean modifier.

7) In the modifer properties, change Operation to Difference.

8) Click the Object field and choose the cube.

9) Click Apply.

10) Move the cube and the wall should now have a hole in it.