Verified answer

Make a new layer and drag below the layer of your photo in the layers palette. Use the magnetic lasso in the tool bar, and click once to start the selection. Then trace around the image with your pointer. Don't need to hold down the mouse button. Go all the way around back to the point you started at. If you haven't quite got it all, you can change in the top toolbar to add to selection, and do again with the bit that was missed. Do Select, Inverse to select the background of the picture. then delete. Put your background onto the bottom blank layer. Ta da! Hope this helps.

It may be an easy answer, but it is hardly easy to do. You have to use various selection tools and methods to "cut" yourself out of one photo and drop into another photo. When done correctly by someone with LOTS of experience and skill it can look really well. When done poorly by an amateur, nothing looks much worse and goofy.

steve