Thread has been merged, doesn't need two on the same subject
Firstly as you are posting I assume you can connect a single PC direct by ethernet to the modem?
To connect up the router, power off everything, swap the wire from the PC to modem, into the router (WAN) to modem, add another ethernet between router (LAN) and PC, power up modem, then router, then PC.
If it doesn't work, are you able to browse to the router's admin pages? Have you got any weird settings in the router from previous use? Try a factory reset on the router, and reboot everything to clear it.
Verify it all works wired, before trying to enable the wireless.
If you still have problems, let us know what router, and confirm how you have it all wired up.