This is an extremely broad question, and as you may have already noticed, we could list hundreds of settings, tweaks, and changes that will "help make [a] server more secure" until the cows come home.
In addition to the general "close/shutdown anything you're not using," and the wonderful recommendations made here already, if this is your first time managing a server, it's also not a bad idea to start working through understanding core security principles.
CentOS is mirrored to RedHat, which is more frequently used in an Enterprise setting. As such, there's a lot of good material out there to get you started. Try taking a look at the NSA's guides to securing Linux:
Understanding how and why people break into servers, and what their attack vectors typically look like is also an invaluable area of understanding (that's always changing)
The final step beyond making sure your server is secure, is ensuring the software and web application you're hosting are also secure. If your going to be doing web hosting, getting a handle on the types of vulnerabilities they pose and what that means for how people can get into your server is key (Hint: SELinux, while a pain, is your friend here). SQL Injection, Cross site scripting, etc - learn these and learn how to run web application assessments. OWASP is a great place to start for that