Is there a good guide to starting up a server on a old PC?

Firstly, do you even have an OS on that machine?

it's just Windows

Though I'm open to using linux instead

Yeah use this.

Ubuntu Server is what people start with, but you should do Proxmox and run Ubuntu Server inside a VM.

I see, is there like a good guide to using these effectively?

At least for Minecraft server usages

Install it, break it, learn why you broke it, lather rinse repeat until proficient.

This is Linux sysadmin at the end of the road, there's no single guide to plug and play, you have choices.

Personally get Proxmox working, install any Ubuntu VM as your testbed and just start learning how to navigate using common commands in CLI. Figure out how to SSH into a host. Once you can do that kind of stuff learn how to download files, learn how to setup Docker, and then visit here https://github.com/itzg/docker-minecraft-server to learn how to run Minecraft inside Docker.

That's kinda what everyone else does, more or less.

Remember to take backups and don't keep them on the same computer, if things implode you will be happy you can restore.

Hmm I see, thanks for giving a lead here.

Feel free to ping me if shit breaks and post any questions you might have. We help here.

Alright nice, I'll ask if I run into something.

Oh and here's a pro tip.
<error message> reddit into google will show you a LOT of people that've had the same shit probably in r/homelab, good to start finding solutions.
Also here, this is a great set of websites to find common tools.

https://github.com/ccbikai/awesome-homelab
https://github.com/awesome-selfhosted/awesome-selfhosted
https://github.com/awesome-foss/awesome-sysadmin

Lots of great free stuff here, sysadmin is a game of learning how they all interact and how to puzzle them together to solve problems.

ahh net, I'll take a look in there

Best of luck. 👍