The default route is the IP address of the next hop when no other routes are known.
To configure the default route to be 192.168.1.1:
ip route 0.0.0.0 0.0.0.0 192.168.1.1
An interface can be used as an alternative to and IP address. To use serial0/0 for destinations not in the routing table, use:
ip route 0.0.0.0 0.0.0.0 serial 0/0