start with a test server, you will learn so much and most likely want to throw away the first attempt anyway.
If you want to stay on the redhat side for testing, you might want to try the this very recent excellent Fedora Guide:
Or you apply for the Developer Subscription if your monitoring is for up to 20 Hosts
The maximum number of hosts monitored with Icinga must not exceed twenty (20).
Or you ask for the “Incl. 60-day free trial” which is in the fine print of Repository Subscription right below the price