first thing to do is to get familiar with using it remember where programs are in the GUI and go this link to learn how to use your command line
http://www.ctssn.com/
then you should read up on IPtables ipchains etc to set up your firewall
and disable sendmail by typing this command without the quote "chkconfig sendmail off" you will probably need to be root to turn this daemon off the link i provided will show you how to switch to root on your box you should try the things they tell you to do you get a better feel for it IMO
hope this helps