How do you start a newsgroup (NNTP) server?

You need to find a friendly node. That’s getting harder these days, since most ISPs have dropped Usenet (largely as a result of UUnet deliberately leasing dedicated T1s and T3s to known spammers for the explicit purpose of flooding Usenet; the spammers were sex site operators in the mid-90s who felt threatened by Usenet, so they set out to burn it down—and succeeded, since the flood of spam meant 90%+ of Usenet traffic was spam, and sysadmins across the world began dropping it). Your best bet is to contact your local university computer department and ask them for a feed, since they’re the ones most likely to still carry Usenet. Problem is, you’ll be getting only a partial feed from them (they’re unlikely to carry any of the porn or binary newsgroups, for example) and you’ll probably need to supplement your feed from elsewhere.

Bear in mind that Usenet can use the Internet for transmission, but it isn’t actually on the Internet. The protocols allow for various sorts of transmission, including phone line and sneakernet. In parts of Africa, for example, Usenet is still mainly transmitted by floppy or CD in a Jeep. You may need to supplement your feed with out-of-band alternatives. If you have a local Fido, they may be able to point you in the right direction.

So… In theory, if I wanted to set up my own NNTP server so I could charge people money to use it I’d be kind of boned?

No, you might even be successful, given how hard it is to find a good node these days. You’d need to make sure you carry the good stuff though — the binary and porn groups. That’s what most people who pay for a feed are looking for. And both of those are very high-bandwidth propositions. I don’t think your profit margin would ever be very high. Just bear in mind that you’re going to have to turn automatic newgroup creation off thanks to the spammers and deal with it manually, which is a royal pain in the ass (and why there are so few sysops still willing to carry Usenet).

Okay. So I want to charge people to use my NNTP server and for that I need the binary feeds. Who would I talk to in order to find a friendly node with which to get that?

My recommendation is to get in touch with your local Fido. The same ones who carry Fido usually carry a Usenet feed. If you’re in a larger city, the local Fido probably holds regular meet’n’greets.

this is like the best kept secret, ever.

not really a secret, but more of a hush and crackdown on the porn. Like @SmashTheState stated it is easy to set up. It’s more difficult to get a good feed that has all you wish to offer.