If the boy is too big for the background
you need to check the resolution of
both pictures. Go to Image, Image
Size, and you will see the resolution
there. Both pictures must have the
same resolution or it won't match.

Download images at the bottom of the
page. (Right click and "Save as")
Go ahead and use the polygonal selection tool. It's also
called the polygon lasso tool. Any tool you use to make a
selection is called a selection tool.

What they call the Radius slider is actually the Radial Filter.
Go to Filters, Blur, Radial Blur.

Note: This isn't how I do it. Give it a try, but I go to Select,
Feather, and feather about 3 pixels or so.