This short and sweet How To explains how to configure the DHCP Client in IOS. This is done using a single command. Go to the interface configuration:
Router(config-if)#ip address dhcp
Some service providers might ask you to use a client-id and/or a hostname of their own choice. This can be done by adding the following parameters to the command above,
Router(config-if)#ip address dhcp client-id INTNAME hostname HOST
INTNAME is the interface name that will be used for the client-id
HOST is the hostname that will be used for the DHCP binding. This hostname can be different from the one that was set for the router in the global configuration.
You can use both of these parameters, one of them, or none of them.
Usually the next thing to worry about is NAT. Search for NAT to read more about how to configure the Inside and Outside interfaces for address translation.
We hope this helps.