Copy a square area of an image using Rectangular Marquee Tool - Learn Psd

First time we take a new document. Go File > New or press Ctrl + N from keyboard.
A window will come.
Set the values like below or as your want/wish

Width: 800 px

Height: 600 px

Resolution: 72 px

Mode: RGB Color

Contents: White

Click OK.

Then a new blank page will open like below

Now we need an image. Go File > Open a window will come now select an image or picture as your wish. We use here a natural image.

Now select Rectangular Marquee Tool from the tools box in the left side.

Create a rectangular shape on natural image. 

Go Edit > Copy or Ctrl+C and past it into new page or blank document.

Select New document then Go Edit > Past 

Finally we get our desire copy a square area of an image like below