CareyHolzman makes arguably some of the best PC repair videos. ifixit.com has repair guides for many phones, tablets, laptops, and other gadgets. You can also search for PC build guides from reputable tech journalists like Linus Tech Tips, simply knowing how a PC is assembled and what each part does goes a long way.

As for tools, a philips head screwdriver is by far the most important for dekstop PCs. You might also want some other screwdrivers like flat heads or torx, or get a milti-bit screwdriver kit. An antistatic wrist strap connected to a safe and proper grounding point provides protection for your components but is not absolutely necessary (I don't have one and I've worked on PCs). You might also want some isopropyl alcohol for cleaning thermal paste (the higher concentration the better for this purpose), and an air duster would be good too (I recommend against duster cans from a sustainability perspective, they contain greenhouse gases. There are alternatives that use fans to blow plain old air). That should get you through the majority of desktop PCs.

If you want to expand to laptops, phones, or other gadgets, you'll probably need some prying picks or spudgers, maybe a heated pad for softening adhesive, and even more screwdriver heads, including various sizes, as well as common security and anti-tamper bits. A magnifying glass might help too, as mobile devices have small components. Check out the tools that iFixit sells, especially the pro-tech took kit if/when you expand from PCs into general purpose gadget repair.