Nagios can be tricky to configure when you've got a good grasp of what's going on, and a complex challenge when you don't. Make sure you read the documentation - particularly the sections on "Configuring Nagios" and "The Basics". Save the advanced topics for when you've got a good understanding of the basics.
Of course, if you don't know where the documentation is, that's hard. I'd start at https://assets.nagios.com/downloads/nag ... nners.html
and if you click on the quickstart installation guide you'll have a bit of a path laid out for you. It sounds like you are past the beginning of https://assets.nagios.com/downloads/nag ... start.html
but there are links to "Post-Installation Configuration" in that document, and even though some of those documents are short they were worth reading, and again, there are links to more.
Not knowing CentOS is certainly going to slow you down, but it's a good choice for Nagios Core since our commercial offerings run on CentOS (or RHEL).
To directly answer your question about plugins, yes, the same procedure applies in a general sense. However, some plugins may require you to install dependencies. For example, https://exchange.nagios.org/directory/P ... us/details
requires ruby, which is not installed by default in CentOS. However, you can install it with yum install -y ruby
You can find instructions for compiling NRPE and Nagios Plugins at https://support.nagios.com/kb/article.php?id=515
. NRPE is by no means required, but it is useful in many cases. If you don't find what you want in our default plugins, you can find many more plugins at https://exchange.nagios.org/
Please let us know if you run into any difficulty.