Hello everybody and welcome to Mastodon!
I know it can be a little confusing at first. But don't worry, it's great and not that complicated to use once you learned the basics.
So here are some tips for helping you to get started.

For posting a message, you just have to type your text in the compose box (it should be at the left of your screen on a desktop) and then click on TOOT.

You can adjust the audience of your message by clicking on the third icon below the compose box.

There is four privacy levels on Mastodon:
1. Public: everyone can see your message.
2. Unlisted: everyone can see your message but it won't appear in the public timelines.
3. Followers-only: only your logged-in followers and the mentioned users will be able to see your message.
4. Direct: only the mentioned users will see it.

Privacy levels are one of the most powerful feature of Mastodon.

On Mastodon, you can add a "content warning" to your posts.
Content warnings are a way to tell people what your message is about. When used, your message will be hidden by default and other people will have to click on a button to unhide it. It is especially useful to warn people about content that can trigger an anxiety or post-traumatic crisis, but can also be used to hide spoilers, etc.
To add a content warning, just click on the CW button bellow the compose box.

Content warning 

When you attach a media (pictures, audio file or video) to one of your post, you have the possibility to mark it as sensitive. If you do so, other people will have to click on a warning to see it.
If you want to mark a media as sensitive, just click on the box below its preview when you are writing your toot.

This is what a picture masked at sensitive looks like.

Mastodon's interface is made of several columns.
The basic interface is made of three columns. The column on the left is where the compose box is. The column on the right is the main menu. The middle column is where the posts are displayed. You can choose which timeline is displayed by clicking on its name in the main menu to the right. When you click on a link, it's also where the content will be shown.

Now, let's take a look at the different timelines.

The home timeline is where the messages made or boosted by people you follow, as well as your own messages, appear.
It's like your own little window to the Fediverse :).

The notifications column is where your notifications are displayed. Here, you can see who boosted or favourited your messages, as well as your new followers and the toots mentioning you.
It's pretty straightforward.

The local timeline list all the public posts made by users of your local Mastodon instance.
For example, since my account is on eldritch.cafe, it will show all the public posts made by eldritch.cafe's users.

The federated timeline is doing the same thing but, instead of only displaying toots made on your instance, it displays toots from all the known Fediverse! Yeah, it's pretty huge. That's why on the biggest instances, this timeline is moving really fast.

The local and the federated timelines are what we call "the public timelines". When you post a post a message with the "public" privacy status, it will not only be displayed on your profile but on those columns as well.

Did you see the little symbol at the top right of most timelines' header? It's actually the column menu!
From it, you can set various things. The columns don't all have the same options, so feel free to take a look by yourself!

There is a lot of things you can tweak to your preferences on Mastodon. To do that, just click on "Preferences" at the bottom of the main menu, on the right side of your screen.

Follow

You can configure a variety of things from the settings panel: your default language, Mastodon's theme (because yes, there are various themes!) and also your display name and avatar.
Most of the options here are self explanatory, so you shouldn't have too much trouble making Mastodon behave just the way you want :).

From there you can activate one of Mastodon's most powerful tool: the advanced web interface, that can display several columns at the same time!
I won't delve into it in this thread, but feel free to take a look around by yourself!

Before I leave you, I thought it would be better to give you some more resources.
Here is the "What is Mastodon?" official video. It explains in the simple way some of the key concepts behind Mastodon. Also, it has cute animations (and that's rad).

youtube.com/watch?v=IPSbNdBmWK

I hope you'll enjoy your stay on Mastodon and I have as much as a good time here as I do :).
And if you have any other questions, feel free to ask around!

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Eldritch Café

Une instance se voulant accueillante pour les personnes queers, féministes et anarchistes ainsi que pour leurs sympathisant·e·s. Nous sommes principalement francophones, mais vous êtes les bienvenu·e·s quelle que soit votre langue.

A welcoming instance for queer, feminist and anarchist people as well as their sympathizers. We are mainly French-speaking people, but you are welcome whatever your language might be.