paul that depends on how the modem is configured by the isp .. if its a ppoe then ul need a username and password if its in a bridged or routed mode then its going to be different... Id suggest that you call you ISP and have them send you an engineer to do the configuration for you. otherwise its going to be hell for you :-) till then play around with your router to get yourself familier... search for basic router configuration examples on the internet.
Hope that helps..