[Trinity]

I've got somewhat of a silly dilemma here...half a year ago I could remove the background from an image and put another one in quite quickly. Now I have a picture that some friends want a new background put in, but I've forgotten how! I'm working on re-educating myself, and thought that maybe someone here would know what to do. I'm using photoshop elements 3.0
Attached is the picture I'm working with.