how to use cisco 1841to implement a WAN and LAN

my WAN connection from the transmit modem connects to the cisco 1841 router via serial0/0/0. the receive modem connects to the router via fastethernet0/0.how do i route properly so that i can use the fastethernet0/1 for the LAN? specifically, i need to route fa0/0 to fa0/1; and fa0/1 to serial0/0/0. how do i get this done please?