Instead of rewriting a step-by-step guide on how to do this, I can recommend two great articles that will get you setup in no time.

First read this article on how to setup Apache and PHP on your Mac:
http://superfancy.net/coding/php-mysql-apache-in-mac-osx-leopard/

Next, follow the instructions from 37signals on using Pow alongside Apache. Make sure to pay attention to the troubleshooting section if you had any trouble.
https://github.com/37signals/pow/wiki/Running-Pow-with-Apache