What a great question! I would say the first things I would recommend learning is Linux in general. It's widely used and an industry standard when it comes to running something on server. A lot of pentesting tools are also designed to run on Linux so one way or another, you'll have to learn you way around a terminal. Tryhackme has a great Linux/Unix terminal learning path for free (iirc).

Secondly, try to understand the basics of programming. Python and Javascript will come very handy for automating simple tasks/scripts. It's also very important to be able to read code to better understand what's going on under the hood. Codecademy and Freecodecamp are great ressources for this!

Third, I'd recommend knowing the basics of network protocols. Udemy is a great ressource for that type of stuff. Understand the HTTP protocol, getting a rough idea of how TCP/IP works, etc.

After entering as an apprentice, work on making yourself processes for when you'll be pentesting. Take notes on what was successful, what was not and you'll eventually start seeing patterns of things that come up often. This will be the stuff you'll wanna start working with when going on a new engagement as they'll often be your entry point into a more serious security flaw.