ModSecurity is a good starting point to secure your web site. OWASP provides the core rule set (CRS) for ModSecurity rules against the most critical web application attack.
ModSecurity is an Apache web server module that provides a web application firewall engine. The ModSecurity Rules Language engine is extrememly flexible and robust and has been referred to as the “Swiss Army Knife of web application firewalls.”
In order to enable users to take full advantage of ModSecurity out of the box, we have developed the Core Rule Set (CRS) which provides critical protections against attacks across most every web architecture.
Check OWASP Core Rule Set home page for more info.