what's the model of your modem? and what's the model of the router?
1. if you can connect to the internet using ONLY the modem and not the router, then that's good for the 1st part
2. check if you have PPPoE(username and password) or DHCP authentication type in order to connect to internet
3.1 if you have a smart, gateway classification of modem,, configure it to bridge mode by accessing the GUI of modem. type 192.168.1.1(default) on webpage address bar and then tweak modem to bridge mode
3.1.1 if on the webpage, it displays page cannot be displayed, open command prompt then type in ipconfig. copy the ip adress on default gateway then type it on the webpage. after setting to bridge mode -> step 4
3.2 if you have a dumb modem and able to connect to the internet -> step 4
4 plug the modem on the wan port of the router. turn on the router. find the reset button on the router then press it for more than 1 minute while its turned ON -> step 5
5.1 try to connect to the internet. FINISH
5.2 if you cant connect ->step 6
6 go to command prompt if it is in vista/windows 7, click start - accessories - RIGHT click command prompt - open as administrator
7 at the command prompt type in ipconfig/release then wait for 1 minute
8 then type ipconfig/renew
9 try browsing -> FINISH