There is a lot of excellent documentation on setting up Nagios, creating checks etc. I don't want to compete with those.
This book contains links I found useful, scripts I created, notes, etc. . It's mainly for my own use but maybe it will help you as well.
System monitoring is not my core business. I'm involved in our (CCE NV) use of Nagios because, as main developer of our application framework, I develop scripts to check that the application is running smoothly.