You want to monitor a single site?
You want to monitor responsiveness of a web application or just of the web server?
You want a continuous monitoring or single check?
What are the acceptable parameter ranges?
But I don't know what sort of googling yo have made, but here is sample you could use:
http://blog.ninethsense.com/site-checker-in-ca-website-health-monitoring-system/[
^] until you respond (first of all to yourself) to the questions above...