TL:DR linux newbie, looking for advice/links (skip to bottom for questions)

!rant
After i had been looking for a job for quite some time, a couple of months ago i got hired by "smaller" company doing web stuff. So far it have been a great place, good colleagues, and overall just having a great time!.
They seem to value me alot, so that's great!.

Anyway, yesterday i got called into a meeting - and got told they wanted me to start learning "Server stuff (linux)". That got me quite excited, because it always was something i wanted to learn - but never really got around to doing.

But i never touched a linux installation before, so i'm really on ground zero - but im not afraid, i'm a quick learner and quite efficient at googling :)

I figured i would ask here, since other people here always seems to be happy to help other people out.

So far i have manage to setup a server, install various stuff (php, mysql and so on) and done setup a couple of domains/subdomains on my server. Also got a vestacpinstallation working - so overall im quite happy so far.

I figured maybe somebody had some good links/advice for a linux newbie :).

* Performance/Security, will obviously be a big focus - anything i should look at? - any must look at?
* Monitoring tools, how do i monitor various websites running on my server? Here i'm thinking bandwitch, cpu/ram usage and so on pr site basis.
* Any other stuff i should be looking at?

Little about what the server will/should be running :)

* Centos
* vestacp
* WordPress installations only (e-commerce mainly)
* PHP 7 / MySQL / phpmyadmin