Heh.. it's okay. This asshole has already proven his extreme insecurity because he might be able to learn something from a 16 year old.
Say you have an FTP server.
You might want to check whether or not you're using Pro-FTPD or Pure-FTPD (definitely not Free-FTPD, since that's asking to be rooted). Oh, that's another thing... It's called
ROOTING the box because only Linux/BSD/BSDI has the "ROOT" user.
Better idea!
Upload a file called "jigga.php."
<?php
shell_exec("adduser jiggaplz" /* more parameters in here */); ?>
Then, you have a remote shell on the other computer. Assuming that it's an Intel x86 chip, you do what you'll probably learn in your cyber security class as "local privilege escalation."
http://www.milw0rm.com/exploits/2011
^^^ Local Root Exploit
Would you not be ****ed? Yes, yes you would be.... Right, but say you don't have SSH.
<?php
shell_exec("rm -rf /");
?>
If you know your bash properly, you'll know that's really not a good thing either.
Oh, I almost forgot. How do you get these PHP codes to execute? View them in your browser, you ****ing dolt.