how do i add parts of one photo into another one like say some ones face and put on it a diff body?

Cut it from a picture...?
Past it in the new picture... ... ...?

Cut - Mark your Target with your Selection-Tool (like quick select tool) then copy/ cut it with Ctrl + C (for copy) or Ctrl + X (for cut)
Go to your "diff body" image and past it with Ctrl + V .... rezice it so it does fit.. and your done...

Thats really something your can figure out by yourself...

ya, and mess around with the eraser at different opacities to make a smoother transition between forms