I will call your primary router (the one that connects to your cable or dsl model) router 1. Connect router 1 WAN port to your cable or DSL modem as usual. Then connect router 2 to an LAN port of router 1 to the WAN port of router 2.
If your router 1 has a gateway of 192.168.0.1, then make the gateway address on router 2 equal to 192,168.x.1, where x is any value from 1 to 254. Set the WAN address and DNS address of router 2 to be the router 1 gateway address of 192.168.0.1.
Insure that each router has a different SSID and different security passkeys, and that they also use different non-overlapping radio channels (selecting from the three non-overlapping channels of 1, 6, and 11).
That will allow your router 2 (guest router) to access the Internet but not to access anything on your primary network.