unless he is meaning to telnet to the telnet service specifically in which case there are guides. such as the one i sent, telnet can allow you to control a machine as though you had local cmd access (almost) ssh is better in my opinion as i said before but otherwise see the link in my previous response
Dave Baldwin
Some of @rgb192's other related questions have been about using telnet to learn about SMTP which is why I answered the way I did.
I had guessed that, based on your post, which was why I included a link about telnet to smtp. It's more "advanced" than the basic telnet tutorials @rgb192 is referring to.
this site has a good tutorial covering most parts of telnet, most tutorials are small because theres not very much too it.
ssh is much better :)