It's incredibly obvious, but usually neglected -- you need to spend less than you take in to accumulate cash.
Most people do not give this much (any?) thought, or at least don't take action on it. So, they tread water. Or worse, accumulate debt.
If you're able to spend less than you earn and bank the difference, your cash position goes up. This puts you ahead of the vast majority of people already.
But, then you can wind up in a tricky position.
Especially once you become good with money, you realize you can turn it into working capital at a pretty good rate. Someone who has spent some time learning the in's and out's of a few marketing channels, or technology, or throwing great events, or... whatever... can then turn free cash into useful production, which turns back into cash, at a good clip.