...at it's simplest.........open the picture you want as the background and the picture you want as a layer on top of that.
....click the second picture to choose that as the one you are working with and either select all or part of the picture and do Edit/Copy........then click on your other picture and do Edit/Paste.......your selection will then be pasted onto the background as a layer which can be worked on seperatel, by opening the Layers work area.
...My advice is get a book...just a "for Idiots" type book would be perfect as PhotoShop is a fine piece of software with a very steep learning curve...good luck. commoner ;-)