Where did you learn everything you needed to know about online discourse?

Online.

in the 80s 'online' was often very local . you learned not to say things you wouldnt say to that persons face because there was every chance you would run into them at some local gathering of users.

Indeed and this very thing happened to me as I was a teenager talking shit to a user on our local MajorBBS system, which was a popular multi line BBS system that had chat, games, forums, fidonet, etc. and he showed up to the next meetup at the park (he did warn me he was going to be there, to kick my ass). Everything turned out fine but lesson learned.
Sounds quite similar to CB radio.

on the internet? it's an on-going process, there is no "everything"

Bingo. Whether or here, or formerly, reddit, I can adjust my style and words to get the upvotes, without compromising my beliefs. But holy shit, people are easy to manipulate.

I'm pretty sure that I still don't know everything I need to know about online discourse.

I am a monkey with a typewriter

By being a part of online discourse as early as 1993 and watching it grow pretty much from inception.

NO TOP POSTING
lol
- I still automatically think "YOUR .SIG IS TOO HUGE!" when reading modern work emails with rows upon rows of name, job title, company, contact info, disclaimers, embedded graphics, etc. at the bottom.
If you count the telegraph as online communication, it'd go back to the mid-1800s.
- FUCK YOU.
  STRONGLY WORDED LETTER TO FOLLOW.
- "Back in my day, we weren't online, we did lines! And there were fuckin' ninjas on the lawn tryna kill us, too!"

I started in a/s/l? chat rooms, but as social media has evolved, so has the discourse. I think I had to relearn every time I changed platforms.

By being terminally online since the previous century.

AOL LOLOL1

ASL?
- 18/f/cali, u? :-)
- I'm pretty sure American sign language looks different than that.

Started with online BBS's in the 1980s where you could get kicked off for being a dick (and your phone number banned) but Larry Wall's "rn" for Usenet used to say before every post:

This program posts news to thousands of machines throughout the entire civilized world. Your message will cost the net hundreds if not thousands of dollars to send everywhere. Please be sure you know what you are doing.

CS 1.6

tumblr

Runescape

The NME (popular British music paper) had a website with a chat room/board that I was active on in the late 90s. I was about 17.

They also gave you free webmail with a @nme.com address, so you felt like a music journalist. Unfortunately, they shut it all down out of the blue in the 00s, with little warning, so I lost my primary email address. Would love to recover all those messages, but they are gone forever.

Over thirty years ago, on TALK.

Chomsky

&totse and IRC mostly

Usenet and IRC, back in the late 90s. Damn I'm getting old.

MMOs and forums.

I'm still learning, despite being around it for over 2 decades at this point.

This year marks three for me. Also still learning.
I'd quote Abe Simpson about being "with it" until they change what "it" is, but that doesn't quite apply online. You're always going to be a couple of steps behind with something as niche things crawl out of the darkness, ever mutating as they turn mainstream.
And to think, when I started on the Internet, I was already technically an adult, but I didn't act like one, that's for sure. Some lessons were hard learned.
Some of it is immortalised if you know where to look. I've changed my pseudonym a handful of times as a result.
- I was in my mid-twenties and my second marriage by the time I really got into internet forums, and they were mostly bicycle related ones. God only know how I'd have turned out if I'd been a teenager or Reddit or the like had been around. I think the closest thing at the time was slashdot, which I looked at briefly and noped the fuck outa there. Bikeforums.net is still alive and kicking, apparently.
  I think the most important lesson I learned early on is that you're not going to change anyone's mind.

AOL 3.0

AOL 2.1 because I was on Mac at the time. Good ol' #macwarez!
- Oh the warez rooms were amazing! That’s how I finally got to play C&C: Red Alert lol

It wasn't just one place. It wasn't even all online.

The starwars.com forum long before the Disney purchase killed it, then Digg until the v4 update, then Reddit until the APIpocalypse.

So many communities I've seen wrecked by corporate greed. The Fediverse gives me hope for people being able to interact online without having to worry about the enshittification.

Alt.flame. iykyk

On MMOs, trying not to sound like a kid, even though I was a kid, so I could join groups of cool people and guilds. Learned a lot of American lingo on Reddit.

Oh man, I am not the primary demographic of lemmy am I?

Hmmm, I think my gateway drug to interacting with actual Internet strangers vs talking online with people I knew IRL, was probably via StarCraft over dial-up in 1998. RuneScape shortly after for developing actual relationships with people, then WoW owned my soul from its vanilla release through the cata expansion.

Several WoW guilds had their own websites/forums, which eventually got me into forums for other topics, then Reddit, then here.

I was going to say AIM (AOL), but really it was Yahoo! Games. Specifically Towers with a little bit of Pool mixed in.

As many others have said IRC and usenet in the 90's

Thank god I was able to learn on the early days of the internet

Discussing online.

Nexopia and NinjaVideo forums.

Yes, I am Unc and chopped.

4chan taught me that, in a highly uncivilised world, the best thing to do is be kind and keep your dignity. It'll either calm people's spirits or infuriate others, and I enjoy both, lol. Also, words don't have to hurt, especially those from online strangers!

Making as many mistakes, missteps, misunderstandings, misconceptions and misses as possible.

Napster chatrooms

Team Fortress 2 custom servers