Hey, welcome to the threadiverse! I'm also a newbie, whose only prior experience with this sort of site has been Reddit (as well as internet forums but they're not quite the same type of thing), so here's what I've figured out so far. (The "threadiverse" is the informal name for the realm of Reddit-like websites linked by federation.)

Apologies for another long post for you to read, but I'll try to make this an easy read. (Feel free to tell me I suck in if you think I wrote this badly or if it was stuff you already knew.)

On Reddit, you have one site, which has a ton of subreddits, each of which is like a little forum and is independently moderated (within the limits of the larger site's policies, of course). Lemmy and /kbin are, basically, like many little "reddits", with a twist: they can talk to each other and so you can be a member of one "reddit" and post/comment on another. Also, subreddits are called "communities" on Lemmy and "magazines" on /kbin but they basically work the same way. You can subscribe to them and see posts from them and post to them, even if they're on other "reddits".

So, yes, you can have your account on lemmy.world, but also subscribe to (for example) /c/patientgamers at sh.itjust.works (which is sometimes written as !patientgamers@sh.itjust.works). Meanwhile, what if you're also interested in the content at /c/patientgamers on lemmy.ml? Well, you can subscribe to that too! (Think of it like being subscribed to two slightly-differently-named subreddits.)

While there's only one actual Reddit which has all the many many subreddits, in the threadiverse there are many "reddits", each of which has some "subreddits". There may be some duplication of more general topics, like memes, but you'll also often find that more specialized "subreddits" are only on certain "reddits" -- for example, my instance, mander.xyz, has a lot of nature and science related communities, not found on other instances. And each of those "reddits" has its own rules, and each of those "subreddits" has its own rules within the instance that hosts it. You'll want to check each out to get a feel for the vibes in each place.

And now for the nitty gritty.

The way this all works is that you basically have two ways to see everything on the threadiverse: (1) on the site where the thing is, and (2) on your home instance. For example, you posted your message on lemmy.world. I can go to lemmy.world to read your post, but I can't reply there, unless I have a lemmy.world account. So how am I commenting? I'm typing this reply to you on mander.xyz. That's because I'm viewing your post on my home instance. I saw your post on the feed of another instance (acutally a /kbin instance, located at fedia.io), and I wanted to reply, so on that page, I got the link to your post ( https://lemmy.world/post/928037 ), copied it and pasted it into my own instance's search bar, and pulled it up on my instance (Mander), and here I am, typing my reply.

Now, I did this only because my instance doesn't already know about your post. I'm not subscribed to !reddit@lemmy.world, which is where you posted this. If I were subscribed, then your post would have appeared in my subscribed feed, on my instance, already. And I'd just view your post and type my reply just like it were a post on my own instance. I'm subscribed to !patientgamers@sh.itjust.works, so new posts there will show up on my subscribed feed.

The first thing I did when I wanted to join Lemmy was that I needed to pick an instance to join. But the second thing I did, almost concurrently, was that I started noticing all the different places that had content I wanted to see. I made a quick list of all those different communities/magazines. So once I joined, I just went and subscribed to all of them.

You can see what communities are on a given instance by clicking "communities" at the top of the page. (Or "Magazines" if you're on a /kbin site.) So I basically just went through the communities lists of a bunch of instances, and checked out what people were posting about, and asked myself, "hey, do I wanna hang out here?".

How do I subscribe? I go to the webpage for the community, like going to the subreddit, and I hit subscribe. What if it's on another instance? I just take its URL, copy it, and paste it in my instance's search bar. Wait a few seconds, then there's a link to the community via my instance. Click subscribe. (Sometimes it's a little buggy and have to go into a post to subscribe. Or it says "subscribe pending" after I click. But, really, I actually am subscribed, and I can tell because those posts start showing up on my subscribed feed.)

Where are my subscribed posts? I just go to my instance's home page (mander.xyz for me, lemmy.world for you) and I can click "Subscribed". Or "Local", which shows posts on my instance. Or "All", which is a feed of all the posts my instance knows about (local and remote). And I can sort them in different ways too.

The search box is surprisingly useful on fediverse platforms, I've found. On Lemmy and /kbin, I can copy the address of any community/magazine or post or comment and stick it in my instance's search box. Wait a few seconds, and it'll find it, and I click on it and do my thing. Sometimes I find posts that my instance didn't know about at all before I pulled them up, so they're "missing" comments that I can see on the post's actual address, but I don't need to see them all on my instance, I just need to pull up the one I want to reply to and post my reply. By the way, these links are that colorful little fediverse star you see beneath your posts. (On /kbin it's in "more" -> "copy URL to fediverse".) Everything has an address and every address is searchable, it seems.

So here's basically how I'm using Lemmy now:

• load up mander.xyz (my homepage)
• check my notifications (which i'll get when people reply to me)
• check my Subscribed feed, and optionally, the Local feed, or even the All feed (if I'm extra bored). anything my instance already knows about is something I can post on like it were local.
• if I want to check out extra stuff on other instances, I can easily just go to those instances and read stuff. If I want to comment/etc., I find a link from there, go back to my instance, paste it in the search box, and do my thing.

Hope this helps!