This. Building a bike from bought parts sounds like great fun, but it's crazy expensive.

Get yourself a second hand bike (we can help you figure out what's right for your needs), figure out each component's function as you pull the bike apart, and buy the tools as you need them. At first you'll want a cone spanner or two, and maybe a crank extractor and appropriate BB tool, but cross that bridge when you come to it; what you end up with might have odd gear here and there.

If you really want to know how to fix bikes, buy one in poor condition, and you'll have a smorgasboard of learning opportunities. You'll want a cheap source of parts, though. Local co-op?

Actually, if you have one of those, you can probably just build a bike up from scratch there. But if your aim is as stated, IMO you should get a well-used quality bike that makes for a good fixer-upper. Think of it a bit like renovating a house... do it in stages. True the wheels, see to the headset and BB, get the brakes and gears working sweet and then start thinking about what else it needs. Fresh drivetrain, maybe new rims, seat, etc...