Oh for the modem,
Just turn it on and plug it to your computer through a ethernet cable as GrueMaster
said.Just fire up the web browser interface like this : open up firefox and in the url bar
put in : 192.168.0.1 and setup what you need to get dialed up and you should be connected to the net.
I have the SpeedStream Model:4100 and I first set it up so it's connected to the net then I just connected my wireless
router to it and bam everything is working.No need to install anything.So really if the modem is already setup
and connected to the net, all you need to do is plug it in through a ethernet cable and run dhcpcd ethX.
Run ifconfig -a as root and find what your interface is called and run dhcpcd yourinterface and that's it.