1- Connect your laptop directly to the ISP modem and verify you have Internet access that way. If so, write down the IP address if gave you. (I am assuming it is via DHCP)
2- Connect the Internet port of the Cisco router to the modem and verify you get a link lights (if available)
3- Connect your laptop to one of the Cisco's 4 Ethernet ports and verify you get an IP address from the Cisco (probably 192.168.1.x) and make sure you can ping the default gateway, which should be the Cisco router.
4- Log into the Cisco router and check to see if it is getting an IP address from the ISP. If it isn't that is where you need to start and figure out why. If it is, use the ping utility and verify the Cisco can ping its default gateway which would be the ISP.
When you have gotten this far, report back and we can take it from there.
Also, here's the link for the manual for that router that I referenced for this reply:
http://downloads.linksys.com/downloads/userguide/1224642019596/WRT54G2_V15_UG_NC-WEB.pdf
Hope this helps