For starters, review The Linux Documentation Project's Guides (at a minimum, if you have a managed service, skim a guide on system administration)
Next, as there are many flavors of Linux with idiosyncratic package managers or processes (RHEL vs Debian, for example) become familiar with the documentation for the flavor of Linux you will be using.
Once you are confident with your skills at the bash prompt and can install, configure, and remove software, make it a point to familiarize yourself (at the very least) with the daemons your server will rely upon to serve your site.
- If you are using an unmanaged host, you should read the user guides (or, where there are none, the manual pages) for every daemon which will be running on the server to ensure a basic level of security and performance (the initial investment in learning will pay off when problems arise)
- If you are using a managed hosting service, you should (at a bare minimum) review the documentation for your web server daemon - e.g. the user guides available for Apache or nginx to ensure that you are able to configure your server to do what you want it to