If your using windows, the easiest software would be pinnacle in my opinion.It won' create the best results, but its pretty easy to use and has the chroma key feature. You may have to pay extra for that feature though... not really sure.
For the background, you could use a picture, but if that looks too cheezy, search for "free stock footage" and if you look carefully, you might find what you're looking for, but most of the time the stock footage won't be free.
Why not just tape a library yourself?