you should make your apache listen on 127.0.0.1:80 and nginx on your external IP:80 (443 if you need ssl)

did you checked the manuals in wthe wiki?

http://wiki.nginx.org/Configuration -> proxying examples

http://wiki.nginx.org/LikeApache-> all you need for a start

after this you should check proxy_cache and different location {} - setups for your static
files.