Usually you have a job and volunteer in your spare time, or you are retired (and therefore have lots of spare time).

You could be employed by the charity in which case they pay you but most charities don’t actually generate enough income to pay that many people. I’d imagine this would be just for the back office to keep the organisation going.

As for how you get involved you need to decide what area you are interested in then just approach local charities and have a chat about getting involved. Most are desperately in need of help so it shouldn’t be too hard.

For peace corps, you get a small stipend.

I do local volunteering; I pay for my own gas and food since that is a pittance and I can afford it.