2 questions about photoshop CS2

1) Im doing a project with multiple layers, and i want one layer to not be able to go over others (meaning if i drag another layer over that one, the layer im dragging will go to the front of it, not the back). How do i this?

2) This is most easily explained if i showed you what i am trying to do.

http://i73.photobucket.com/albums/i212/Thoto1/3hk47_print.jpg

I am trying to make it so the droid featured in that picture is infront of an all white backround, and not a dark blue. The shadow isnt required. How would i go about doing this?

Thanks in advance.

in the layers pallet simply make sure the subject is on a layer "above" the white background layer.