A Newbie Question

[reggie]

hi,           I have a Redhat machine 6.2 im running sendmail, bind, squid, httpd, etc... and a cisco router 2520. My question is where should I start putting on my security??? and what security measures should I run to protect my machines from attacks.. and malicious users. BTW im using acl in my cisco but I need more details on this... anybody have a very nice link???     regards   rym