Depending on the operating system. More than likely you would be utilizing Linux as a base platform, unless you'll be building your application using .NET as a language. You'll need advanced knowledge of maintaining a Linux based system utilizing nothing more than the command console (as it does not come with a desktop environment and it is not recommended to install one on a server). You will need advanced experience installing Apache, PHP, MySQL (or MariaDB as it is better) and any other software and modules that you may need for your application to function. You will also need knowledge and experience in networking, how the internet works, internet security, etc. You will need to know how to install, configure and maintain a firewall. You will need to know how to make routine updates, run security scans, patch any vulnerabilities, etc.
Oh, did I mention, you will need to do all of this via. a command line console?
All in all the above sounds pretty scary, and as a seasoned systems administrator even I cringe when I read it. However, in the end, you'll never learn any of the above without any trial and error. What I would recommend is buying a cheap computer to practice with, have it set up in your home (don't actually run your website off it publicly though). If you're unable to do that and your compter is decent enough, you can get a tool for your computer that you can run virtual desktop containers and practice within one of those. If you need tips, Google will be your best friend, otherwise, you can ask in here and someone will be more than happy to help you.
