Learning an operating system properly means going past the install screen: understanding the commands, the settings, and how the system actually manages your machine. These are two channels I keep in my list for exactly that, one for Linux and one for the Windows side.
Riba Linux — Short, no-nonsense installs and overviews of just about every Linux distribution out there, plus practical how-to videos. The fastest way to see what a distro actually looks and feels like before you commit to it.
MSFT WebCast — The Windows counterpart. Beginner-friendly walkthroughs of Windows Server, networking services, and Active Directory, aimed at anyone managing a Windows environment.
If you only take one thing from this: pick a distro or a server edition and actually install it in a virtual machine while you watch. Reading about an OS teaches you very little; breaking one and fixing it teaches you a lot.