Experience
What can go wrong? Broken parts, damaged frames, failures while riding, etc.

Videos can be useful for some projects. But my first place to check is the Park Tool Repair Help pages.

For instance, the "threadless" headsets, where the stem clamps around the steerer tube, is easy to adjust, once you understand how it works. (The two bolts on the stem are loosened, then the top cap is tightened "just enough" to preload the bearings, using very light finger pressure. Finally, tighten the two bolts to hold it all together. It was a "Oh, I see!" moment after I read the Park Tool guide.)

And I've seen a lot of posts over the years with riders randomly turning the limit screws and the barrel adjusters, trying to fix their derailleur shifting. Don't do that! It's easy to adjust correctly, once you follow the steps (and a repair stand makes it way easier.)

Even replacing a chain is more complicated now than 30 years ago. The chain pins are just about flush with the sides of the chain, so the correct assembly is critical.

I recently got stuck after I pulled out the brake cable that threads through my frame. I assumed there must be a guide channel inside, but no. The bike store got me going, and now I know the method--see this post. I should have done some research first.

I've seen lots of advice here on BF about torque, saying "you just need a feel for it". But I use a torque wrench on critical bolts, instead of using "years of experience". (I've adjusted seat post clamps often enough, and those I do have a feel for.)

Home mechanics have cracked their frame by clamping it too hard on the repair stand. I always clamp the seat post.

I guess my point here is to do at least a little research if you are starting a new project. Most everything is pretty easy once you know the methods.

Tools
I posted a list of the tools I've accumulated over the years. (It's for modern bikes, vintage bikes will be different.) I buy them as I need them for new projects. Good tools do make a difference-- quality hex wrenches work much better, for instance.

But I've gotten a few tools at Harbor Freight: some larger metric sockets that I only use occasionally, and some adapters to fit different sockets together.