how can I take a background from one picture and make it the background of a different picture?

Use the selection tools to select the background, copy it, in the other document paste it. Then move layer below the image so it is in the background.

Two things that can hinder this, first the background could have a hole in it due to the foreground objects. You will need to fill it in using cloning or healing tools.

Second the second document needs to ha a transparent area to see the new background. Blend modes, masks, etc can be used to hide what you don't want so the new background will show.