How to drag another picture to the background?

Maybe I'm such a fool *aplogetic bow* and i really did look through all the possible tutorials. But I could not find any thing which could answer my question *sigh*

sooo, pls forgive me if i post a question which might be easy for you, seasoned photoshoppers. but i'm quite new to photoshop, using Photoshop CS.

all right, i open a background, clean white. after decorating it with stamps all those, then i think i want to put a guy on this background. so i go open a picture of his.
then i try everything to get him onto the background. "select", "free transform". "copy and paste" dun seem to work for me. then i tried another layer with the background copy then tried all those the same things. but i can't. whyyyyyy is it ?? and pls tell me any possible solutions/methods?

thanks for your patience! gomen!