YouTube is really poor advice these days for the simple reason that anyone, regardless of competence, can put up a video and there is too much conflicting, confusing and just plain incorrect information among the good advice - I know how to fix bikes so I can pick the difference, for someone learning, stay away from YouTube.

When learning, it's best to use a single source because multiple sources just leads to confusion.

As suggested above, Park Tools are probably the best internet source, mainly because their advice is sound and they cover just about everything.

Once you've got some experience, then you can go playing the YouTube game, if only to laugh at some of the inane advice (though you do learn some good stuff to from time to time).

The other thing to remember is that bikes are pretty basic and forgiving beasts and there's not a lot you can stuff up so badly a bike shop won't be able to unstuff it for you. Best to learn on a cheap bike though.