Do NOT use telnet. Use SSH (Secure Shell) instead - it has already pretty much dethroned telnet as the standard remote access application. SSH uses, unlike telnet, encryption so passwords and other sensitive information cannot be sniffed across the network.
Your machine is problaby set up to allow incoming SSH connections already. Try accessing it using
ssh user@host. So I access my server as
ssh hw@blaha.com. The IP address works as well as the hostname so use that if the computer doesn't have a usable DNS name.
You may have to configure the built-in firewall on the server to allow incoming SSH connections in case you're running one. Most distributions provide easy-to-use graphical or non-graphical programs to configure the firewall software.
Håkan