The jack on the back is either for a network adapter (NIC; RJ45 - like a telephone slot only bigger) or a modem (telephone slot- RJ11). If it's a modem you can't plug it into the router. If you a NIC then you can router it like your other computers.

Alternatively, get a wireless router with wired slots and do both. 

Ok so your saying I can hook it up like this?

Modem/Cable -> Wired Router -> Wireless router -> Notebook connection

The wireless router will have 4 ethernet ports (just like you present wired router).  Just replace your current router with the wireless router.  You can then connect up to 4 computers by a wired connection and up to 251 more by wireless.  If, in the future, you need more wired ports, then you could hook it up like you suggest, but if you do, be aware that one of the routers will have to be configured as an access point, rather that a router, because you cannot have more than one router on the same network.

Steve

As above,

connect the laptop to one of the spare ports on the wired router.

Otherwise buy the wireless router with the wired ports and run your laptop on the wireless conection.

Do not connect two routers in series.

Hope this helps