Well, I've got a boring answer for you today :D

Honestly the best thing you can do first is probably to learn accounting (and maybe tax law?). A business selling computers is a business first. I use GnuCash, it's very good enough. It's not that hard to learn from the manual:

https://gnucash-docs-rst.readthedocs.io/en/latest/guide/C/ch_basics.html

In terms of choosing products to sell? You can't compete and win with large companies -- you will only lose money. You can only create a new game that you can win at. For example, specializing in something specific -- retrocomputing, DIY kits, weird cooling, or maybe high-end commercial hardware produced for the Asian market, but sold to the US prosumer market. For example powerful embedded routers for hotels make fantastic home routers. I have one running OpenWRT and it blows any US-made consumer stuff out of the water.

Overall I'd choose an 'evergreen' product -- something neat from Asia that doesn't get obsolete fast at all -- that's why I chose routers as an example. Very generally we get a lot of neat stuff in Asia that you don't. "Store that sells cool stuff from Asia" sounds like a lot of fun to run :D

That brings me to the third thing -- establishing supplier relationships is pretty important if you're buying products.

Finally, B2B is way easier to make money that B2C (and less time consuming and more chill). So if you look at my commercial router example, you've got a cost advantage, it's a good product, it doesn't get obsolete fast, and some businesses need quite a few of them. So setting it up with some security cameras sounds at least like an OK 'lifestyle business', although maybe too boring for a hobby business.

Sadly I can't think of any solid course on these last 3 things, maybe there's a 'small business 101' out there somewhere.

If you're running a repair shop from home, the economics are bit easier, as you don't have to source product, just some tools. You need some decent Chinese tools (ping me and I will remember good brands for you), and some experience using them. It requires a lot of specialized skills, and doesn't make much money, but can be a lot of fun and can make a difference in people's lives. Learn at least how and when to desolder and replace capacitors, how to replace a laptop screen, and a bit of data and password recovery (personally I've required photo ID to do this last one). A good way to get started is to buy broken stuff and attempt repairs. Avoid microwaves, CRTs, and mains-power in general until you know how to deal with these safely.

Learning to repair electronics is an uphill battle these days. Most things are not made to be repairable. In a sense, that's what creates demand for your business if you can do it anyway. It requires a lot of creativity and knowledge, so there's a lot of cool stuff to learn. Learning to build electronics is as good a place to start as any, I guess. Adafruit and Sparkfun are good companies that offer lots of introductory material:

https://learn.adafruit.com/guides/beginner

https://learn.sparkfun.com/tutorials/where-do-i-start/all