BASIC Router configuration - Cisco 881

I have a Cisco 881 I am setting up on a cable internet line. My client has a static IP address and a cable modem that provides an RJ-45 connector to the WAN interface on the Cisco. I have IP addresses configured on both the internal and external interfaces and both are accessible from their respective sides, however the router is not passing packets between the two interfaces. Along with configuring routing properly, I also need NAT port forwarding to a specific address on the LAN. Router configuration to be performed via SSH.