So when adding an account, I would just add to that wall of postings? How would people interact on such a platform? I am coming from Usenet, which had the greatest user interface ever (IMHO): you just follow-up on a posting and replies will pop up in your stream of unread messages. How would this work on Mastodon?
You set up your "wall of postings" (the "feed" or "timeline" in the jargon of socials) by following (subscribing to) the people you're interested in. On Mastodon you can also follow topics by following "hashtags", which are the sets of posts by any user tagged with a string preceeded by a hash character. For example, following the hashtag #retrocomputing will bring in your timeline the posts about that topic.
So, on socials, you typically scan your feed and the feeds of any additional hashtags you're interested in. For each post you can reshare it, reply (comment), or like it.
Some additional resources: 1) https://opensource.com/article/23/1/mastodon-beginners-guide 2) https://github.com/joyeusenoelle/GuideToMastodon
The only software you need to get started is a web browser to use a web client, which is typically the Mastodon server you create an account on. Picking a server is the only potentially confusing choice, so for retrocomputing enthusiasts I recommend creating an account at https://bitbang.social or https://oldbytes.space Once you have an account on a server you can migrate to another one if needed.
For more on Mastodon see https://joinmastodon.org For any other questions feel free to ask here or follow me on Mastodon at @[email protected] and ask there.