Step 6: Play with the device you connected to
One of the most easy thing to hack via telnet, is a Cisco router. All Cisco routers have a telnet server built in. To learn the Cisco "language" go to this website.
When connected to a Cisco router you will get something similar to the picture below. Type "enable" and you can use the privileged mode. After entering privileged mode, type "config t" to configure the router.