This is all covered in the manual for the router. As long as you don't have any weird routes or port forwards or whatever set up, all you need to do is reset it, browse to it from one of the PCs (should be 192.168.0.1) enter the default credentials (admin for user and blank for password, possibly) and then set up the WAN side of the connection (DHCP for residential cable, PPPoE for residential DSL, and static otherwise). From here, you configure the WPA/WEP/WPA2 key and password, which is outlined in the manual.
Lastly, configure the password for accessing the router itself, which as mentioned above should be blank by default. Again, look at the manual.