This chapter of the Googles SRE book gives you a good overview.

https://landing.google.com/sre/sre-book/chapters/monitoring-...

But I think it is less abut what specific tool to use, but more to just get started with one and learn how to understand your production system behavior and dependencies from metrics and graphs.

An easy way to get going for monitoring an application is using a hosted solution like newrelic.com Their free version should be sufficient for a very long time.

If you want to run it yourself there are opensource solutions like prometheus.io or riemann.io among many others.