Can't recommend anything that's not already known, but the thing that helped me the most is building projects. I recommend you start a homelab.

A homelab needs hardware. I don't know what kind of computer store you want to make, there aren't any around me, but I imagine it will include some sort of hardware maintenance. Get yourself a couple of broken laptops or PCs, usually sold for very cheap and try to diagnose the problems, order parts, install them, troubleshoot them. If at any point you feel lost, use your favorite search engine. You will probably land on some Wikipedia page. Read through it, and if you don't know a word, search for it. Repeat this recursively and your knowledge will kind of build itself :)

This hardware will probably be pretty old unless you spent a lot, so try to upgrade it. Get some cheapo SSDs, RAM, etc. I imagine customers would need a service like this.

A homelab may be useless without software. I had the most difficulty setting up and provisioning Windows (I've been a Linux admin for God knows how long), so since you'll have a few working machines, install Linux on at least two, install Windows on at least two (of course use something you have laying around as well), so that you can try out different OSes and ways to communicate between them. Now you have a home lab :)

On Linux, the skills I needed the most to provision my own servers (off the top of my head), disk management (mounting/unmounting volumes, formatting, partitioning, etc.) working with services (searching for "systemd" and "systemd service" should yield very good resources), basic UNIX shell utilities (cp, rm, mv, etc.). Linux man pages are also your friend. I imagine you probably won't be working with servers a lot, but there is no better way to learn Linux IMHO. Run a web server and some sort of file sharing server, such as Samba.

From the above, learn the equivalent on Windows + Active Directory. This is where you'll see your knowledge celitify.

Network them. Get a switch that supports VLANs, I recommend older enterprise switches, such as the Cisco Catalyst 3xxx or HP Enterprise switches, which you can get for cheap. They use a command line interface for configuration, but the guides for it demonstrate a ton of key networking concepts, which you will definitely find helpful when diagnosing problems for a customer, trying to imagine their network layout. Here, I recommend NetworkChuck and David Bombal on YouTube. Again, if you don't understand something, search on the interwebz, applying the recursive method mentioned above. Then run Wireshark on one machine to scan the network traffic and search for anything unknown.

I know I went a bit too far, but once you build a homelab, you will be able to fix at least 90% of problems people encounter with hardware, software, networks, because you'll naturally build a thorough understanding of the systems and networks your customers have at home and even be able to replicate them.

Hope this was helpful at least somewhat, and sorry for the long comment. If you need help, feel free to reach out to me or any other admin community, we're all happy to help :)

Wish you the very best!