hrm hrm, you seem to be a reasonable if clueless person. You'll learn what and how and what NOT and how NOT to ask a question after awhile.
Anywho, think of a port as a program. So their tftp port is open. Great - wonderful. It can be quite handy - but only if you know how to use it. Telnet commands... psh. There isn't any "Telnet Commands". All telnet does is allow you to communicate with a program on another computer directly and in the raw. This communication follows certain rules and guidelines so that you may understand each other and perform the desired tasks.
So how do you communicate with tFTP? Well, go learn how! How do you communicate with port 80 (webserver)? Well, go learn how! You see, we can't tell you how to hack. And really, nobody should be taught how to hack. Hacking should come natural after you know all about something. For instance, if you're a lock maker, you know how to pick locks! Nobody taught you how to pick locks, you didn't ever have to learn how. You just know! Cause you understand locks THAT well.
So now's the time to start learning how to communicate with various programs. Once you've learned how, you can start experimenting with ways to break into their security - or see the signs of weak security. When you read about exploits written by people who know a heck of a lot MORE than you do about some program - then you'll understand how to use it - cause you too is familiar.
So for now, don't worry about learning how to install a trojan on somebody's computer through a port. Instead, learn how to request a webpage from port 80 (HTTP), or learn how to download a file via 21 (FTP), or learn how to send an email with port 25 (SMTP)...
That's the knowledge that is important. Come back with that question after you've learned more protocols than I have. >
SMTP, POP3, DNS, SOCKS 5, HTTP, FTP, TELNET, IRC, IP, TCP, UDP, ICMP, WHOIS, FINGER, IDENT..... um, I think that's pretty much it.