How can I set up a desktop on my DietPi machine that's always up, then remotely access it on my main Windows machine? I'm very Linux illiterate and everything I touch breaks, so I'd like to slowly learn with a desktop in a way that kinda makes sense
First, install the OS.
Then, install SSH and configure it. Watch a tutorial for it, it's not the simplest thing in the world. If you need not only terminal but also graphical access, then install and configure a VNC or RDP client, the tutorial is probably going to cover this though.
If you just want to learn linux though, it makes more sense to install it on a virtual machine in your windows system, or even on a separate partition.