so id say try to dive into some rooms on picoCTF or tryhackme or hackthebox. if you get stuck, you can look up a writeup, and that writeup will probably be useful for learning skills you can apply in other rooms
but for fundamentals, i would say start with the fundamentals of either linux or windows server, if you have time try to set up a linux VM or windows server VM and try to operate them as an actual server