Capital generally means accumulated savings/investment. One can still have cash without having capital, and this is actually pretty common (or even people having negative capital, like student debt).
In short, one buys things by doing/making things for people who do have capital, or for people who don't have capital but have themselves acquired money by doing things for people with capital.
If that was as simple as you write, then we wouldn't have unemployment and there would be no need for welfare - after all, you don't have to go hungry when you're jobless, you can always "do/make things for people with capital or" someone like you closer to them in the chain.
We have unemployment because of minimum wage laws. If the minimum wage is n, then many jobs paying less than n are eliminated, as the potential employer might decide that they'd rather go without the service than pay n. Developed countries with lower minimum wages like Singapore, Hong Kong and Taiwan have unemployment rates under 4%.
In short, one buys things by doing/making things for people who do have capital, or for people who don't have capital but have themselves acquired money by doing things for people with capital.