PC:
Open Paint.
Go to the window you want to take a picture of.
Press "Print Screen" or "Prt Scrn". (You don't have to print anything!)
Go back to Paint.
Press Control and V, together, like you're pasting text.
The window you took an image of should appear.
To edit, open Paint, Photoshop, or some kind of photo editing program, open the image, and do what you need.

Mac:
Go to the window you want to take a picture of.
Press Alt, Shift, and 4, altogether. If you have the volume up, you should hear a little sound like a camera click.
An image should appear on your desktop labeled "Picture 1".
To edit, open Paint, Photoshop, or some kind of photo editing program, open the image, and do what you need.