CISCO 1720 Router

here's how you connect to the console port

http://www.cisco.com/univercd/cc/td/doc/product/access/acs_mod/1700/1700qsg.htm

In case you can't find the console cable, you can telnet to the router using the ip address of the LAN interface. This will probably appear as the LAN default gateway. The router may be password protected. At least it should be if it was set up properly. You will need the telnet password and the "enable" or "enable-secret" password to perform configuration changes. If you don't have those, you will need to do a password recovery procedure. Search Cisco for your model and the words "password recovery". However, that typically requires a console connection. A normal connection looks something like this:
routername>
To get to configuration mode, you type "enable" and if the router is NOT password protected, you get this:
routername#
Otherwise, you are prompted to enter a password before the router will allow you to connect. Once in, Cisco routers have context sensitive help listings. That means you can enter a "?" and find out what commands are available for your current position. You can also use it to find out the options for a command by typing the "?" at the end of your partial command. Hope this helps.