Best command is
"netstat -tap|grep LISTEN"
Thats gives you the list of all servers listening for connection requests from the internet.
Also, you can use GUI's (gnome-network-conf or knetmap or something along those lines is the command to the gui that has a netstat menu u can use in case u don't want to read man pages or remember a whole bunch of flags)
shut off unnecessary services using kill (make sure they're not set as symlinks in /etc/rc.d/rc<whatever>.d/ or else they'll start up again the next time u reboot). U can use GUI's to do that too (like tksysv or ksysv).