Using the Select tool, select the area of the picture that you would like to cut out. If you would like to cut out an area of the image that is irregular in shape, e.g., you want to separate a figure from its background, change the selection shape to "Free-form selection" in Paint version 6.0 or newer, or use the Free-form tool in versions before 6.0.
With the selection made, press and hold the "Ctrl" key on your keyboard. Then, press "X". This cuts the selection out of the current image and stores it in the clipboard. The image you are working on is updated with a cutout of the selection to signify that the cut was made.
As long as you do not cut or copy anything else, the cut-out image is retained in memory. Paste it back into Paint or any other program capable of handling bitmap images by pressing and holding the "Ctrl" key on your keyboard and pressing "V". You can paste multiple copies of the cut-out image without having to repeat the process described above.