If the switch that you bought is an unmanaged switch, all you have to do is connect the switch to the CGN3 with a Cat 5e or Cat 6 cable, and connect your PCs to the gigabit switch. You should see a yellow / orange port LED at the back of the CGN3 for the port that is connected to the switch. Anything else that you have that also uses a 10/100 Mb/s port should also be connected to the switch. That will keep all of the 10/100 Mb/s devices isolated from the CGN3. You should also be able to see the switch LEDs display a different colour for the 10/100 mb/s connected ports versus the gigabit connected port.
If the switch is a managed switch, then you will have to take a read thru the switch manual and log into the switch to change the settings if necessary. What model switch did you purchase?