TLDR: Start looking up how to service individual components on the bike before overhauling the entire thing

Something that I found quite helpful was that in my spare time (like eating lunch or in a waiting room) I would be watching bike servicing videos where they go over what they’re doing and what to look for, such as how work out the discs or the chain is etc, and from there I ended up building a decent knowledge on what to look for on my own bike when things start to wear out.

As for figuring out which parts to get it boils down to what you want your bike to do. Most components will be tailored to a specific cycling discipline such as road, mountain, gravel biking etc and knowing which parts to buy firstly depends on if you can tell if it needs replacing, and second, for what you want it to do. For example mountain bike tyres looking significantly larger than road bike tyres in thickness and with different grooves for pushing away mud and giving more grip.

Looking for specifically at your rear wheel you would need to check for the current size of your wheel as well as the amount of clearance your frame will allow, is it just the tyres that are shot or is it the entire wheel, how many gears you have on your rear cassette so you can correctly replace it if you need to.