I wrote this short "guide" in response to a new user earlier and was debating turning it into a full post so I'll put this there for the time being.
The home page / feed
When you are on the home page of your instance, above the feed you will see this:
posts/comments: chose if you want the feed to show you posts or comments of posts.
eye/barred eye: chose if you want the feed to include hidden posts or not.
subscribed/local/all: choses from where the feed should pull what it shows you, subscribed to have it show you only content from communities you are subscribed to, local if you want it to show you content from only your home instance including comunities you aren't subscribed to, all if you want it to show you content from every instances your home instance hasn't defederated from.
And finally the sorting menu: It allow you to chose the order in which the feed should display content and include the following options
- hot - sort based on highest upvote/downvote ratio and time of the latest comment and time of the post
- active - sort based on highest upvote/downvote ratio and time of the latest comment
- scaled - sort based on highest upvote/downvote ratio and time of the latest comment but boost posts from smaller communities
- controversial - sort by how close are the number of downvotes and the number of upvotes
- new - sort from newest post to oldest
- old -sort from oldest post to newest
- most comments - sort by number of comments
- new comments - sort by how recently a post has been created or received a new comment
- top [period of time] - sort by highest number of upvotes among posts made during the last specified period of time
You can change what these are set as by default in the settings menu.
Posting
When you click the button "post" you will see this:
From top to bottom we have:
- Title - self explanatory, the title of your post
- URL - if you want a link to some other site on the internet in your post, put the link here
- image - if you want an image to appear next to your title in the feed
- image URL - same as image but with an image URL link instead of uploading a file
- Body - the content of your post, typically text but can also contain GIFs and images
- language - if you want lemmy to precise in which language the post is written in, rarely ever used in practice
- community - the community that will receive your post
- NSFW - click this if you want the associated image of your post to be blured on the feed, use it for anything the require a content warning
Each posts has these icons: 
From left to right:
- the number of comments the post has
- link to the post
- link to view the post from it's home instance
- de-format the markdown syntax to see the raw text (I talk about markdown after)
- favorite the post
- crosspost, post a copy of the post to another community with automatic links to the original
- additional options like block a user, flag a post, etc
Comments have these icons: 
- deformat markdown
- respond to the comment
- favorite the comment
Markdown
Markdown is a syntax system built in lemmy that allows you to modify your text in various ways:
You can make very big text for titles
of different sizes
italic bold ~small~ ~~barred~~
- make lists
Site quotes
write code
and post images 
Additional pro tip: you can turn any image into an emoji
Remember the button to de-format the markdown syntax from the previous section? If you click on it you will see that all this funny stuff I just did correspond to this text:
# You can make very big text for titles
#### of different sizes
*italic* **bold** ~small~ ~~barred~~
- make lists
> Site quotes
`` `
write code
`` `
and post images 
Additional pro tip: you can turn any image into an emoji 
Federation
Federation is a part of the fediverse that tend to confuse peoples, because it is confusing.
Federation is made possible by a communication protocol called activitypub, and is basically a way for different sites to "talk" to each other kind of like e-mail or RSS. I absolutely wouldn't be able to explain how it works from a technical standpoint, all I know is that it allow sites to exchange information like "such post has been made on instance such-and-such" "such user from instance whatever has liked random post from instance what-s-it-called" in a decentralized spider web kind of way.
It allows not only every lemmy instance but every instance of every fediverse software, from mastodon to peertube to exchange content and data (unless they don't want to). This means that in theory a mastodon user can like and comment a lemmy post and watch a peertube video (in practice it's a little bit finicky and you have to try multiple times to understand how to do it).
Now let's talk about de-federation. De-federation has this strange and confusing one-way of functioning. It doesn't prevent the instance you de-federated from from requesting information from your instance, but it does forbid your instance from requesting information from theirs, in other words, when you de-federate from another instance your users won't be able to see the other instance's posts, comments and likes but the other instance's users will still be able to see your posts, comments and likes unless they de-federate from you as well. For example, from lemmygrad, we can still see lemmy.world comments on lemmy.ml posts and we can upvote/downvote and comment on them, but the lemmy.world user won't see any of it.
The wider fediverse
Since you're already on it, I suggest also taking a look at other fediverse apps. You can easily find them by googling fediverse. There is mastodon/miskey/akkoma/etc which are twitter-like micro-blogging softwares, peertube a video sharing platform like youtube, and many more.
The Marxist-leninist community on the fediverse is most well established on lemmy but we also have instances of other fediverse software. We have a mastodon instance established by an Hexbear user and a peertube instance established by lemmygrad user tankietanuki, I recommend both.
