assuming the router has been set up to the default (obtain IP automatically, DHCP server enabled), then connect the WAN socket to the modem & one LAN socket to the PC using ethernet cable.
Now switch everything off. Then boot the modem & allow it time to settle down (synch & ready lights steady), now turn on the router & again allow it to run through its boot routine. Now boot your pc - you
should get an IP and be able to connect.
Once the main pc is ok, you should then go on to connect the USB adapter - make sure you set up the security.
<edit> too slow