If your eye got caught by the waves of Mastodon-is-like-Twitter-but-much-nicer articles, you may find these notes marginally useful. I’m going to assume you’ve got as far as knowing Mastodon is not the whole network, but a microblogging platform integrated into the wider and older ‘Fediverse’. If that’s confusing already, see here for a primer (and maybe here but ignore the implication that Mastodon Social is a GNU Social instance).
If you're looking for microblogging then your choice is largely between Mastodon and Pleroma instances (although Misskey is looking interesting and GNU Social/PostActiv haven’t gone anywhere), but there are other bits of Fedi as well: recent and work-in-progress additions include Peertube for federated video and Pixelfed for image posting. (Right now Pixelfed has partial federation support but following and anything else involving account inboxes won’t work, and Peertube following has complications arising from the way it replicated YouTube’s user/channel distinction.)
If you’re possessed of the perverse sort of mind that that intrigues instead of putting off, then you’ll be looking to choose an instance to join (unless you intend to host your own, in which case you are indeed on your own). There are various instance lists, such as fediverse.network and those at distsn.org, but the main thing to grasp is that this is rather like having a large extended network of friends and colleagues, some of whom aren’t on speaking terms with each other.
In a sense, this is a feature: it’s not that Fedi has any fewer nasty people (for anyone’s definition of nasty) than any unfederated social network, but that the federated aspect lets people sort themselves into local instances that suit their attitudes and interests, including the level of blocking: you can join anything from a free speech instance to an instance that administratively blocks free speech instances so its users never see posts from them. However, if you don’t want to make more than one Fediverse account (and many people do make multiple accounts to suit the local interests of multiple instances), and you want an unrestricted view of the Fediverse, then you can end up specifically looking for an instance that’s lightly moderated (so it won’t be blocked on sight by restrictive instances) but with a no-admin-level-block policy. (You can block/mute both individuals and instances on your own account.)
In particular, mastodon.social, the big flagship Mastodon instance, has a reputation for blocking large chunks of the Fediverse without telling anyone it’s done so. Edit: this has now changed after, er, events took place. (I’m not sure whether this covers all admin-level blocks or whether individuals may also have been blocked/muted.)
Also, if you’re academically inclined and considering a sign-up on scholar.social, check the blocklist/mutelist (it silences unmoderated instances, but at least has a public list of those it’s silenced) and also read this, this and this: the admin’s conception of academic freedom is, er, not uncontroversial.
While we’re on the jolly subject of blocklists, there are a couple that are passed around among some admins of more block-happy instances, and, well, I’m not going to tell you what side to take here.
When you look at the public front page of an instance, you may find yourself viewing the ‘federated timeline’/‘the whole known network’, which contains every post the instance knows about via subscription or relay. However, when you’re deciding which instance to join you’ll usually want to check the local timeline instead: if necessary you can get an instance’s most recent posts from here. (Remotely viewing local timelines is a semi-solved problem: the Mastalab app very recently added remote instance subscriptions of this sort.)
(Some instances have ‘followbots’ that follow as many people as possible in order to give the fullest possible view of the Fediverse. Which is why, if I want to do a hashtag search, instead of bothering with the limited view of the local instance I may go to https://mastodon.host/tags/tag-goes-here in order to take advantage of its followbot. There’s also a tag explorer tool here, and if you can follow a Japanese UI there’s a Fediverse search tool here. Let’s say federation has its complications. Mastodon’s search features are also deliberately restricted, the reasoning apparently being that they’d be used for harassment if they were more useful.)
If you want to interact with a post you've found via the open Web but your instance doesn’t know about it yet, you can try putting its URL into the search box in the Mastodon UI, or the Mastodon-style front-end available in Pleroma (if there’s no login option to use the latter, try adding /web to the end of the hostname). Mastodon can generally find other Mastodon posts unless a server’s configured in a way it dislikes. Pleroma has a set-up where a remote URL with /objects/ in it redirects to a local one with /notice/ in it, and the latter doesn’t always seem to return results in Mastodon. Friendica URLs don’t work in Mastodon search and I don’t know of any work-around.
Misc. Mastodon vs. Pleroma stuff: Mastodon has a character limit of 500 unless an instance changes it, Pleroma has 5000; Mastodon’s post privacy settings may not be available to set in Pleroma’s native front-end unless the admin has turned them on (though none of them provides real security: it’s more like whispering when in a crowded room); the native Pleroma front-end has no favourites list or ability to search for anything but usernames, but the Mastodon-style front-end can be used instead; if Pleroma won’t load older posts, or displays sensitivity banners when they’ve been set on images even though you turned them off, switch timelines and then switch back; I’ve had random problems following accounts from a Pleroma instance and never receiving posts from them, but I don’t yet know whether it’s instance-specific; and Mastodon is useless for heavy users trying to follow what happened overnight, because ‘I figured nobody is likely to scroll back 400 items’.
Culture-wise, according to legend Mastodon is a fluffy cloud heaven of sweetness and light, while GNU Social and Pleroma instances compete to have the shiniest jackboots. Having viewed the federated timeline the night McCain died, I can confirm that Mastodon has plenty of horrible people who will be openly vile when they feel they have political/social licence to. There is some truth in the ‘social network without Nazis’ thing, but...
Basically, the nature of federation means generalisations about culture are seldom very useful once they get wider then the culture of specific instances, but Mastodon by design tries to foster niceness and that often does, indeed, foster enforced niceness. The Content Warning feature (actually a repurposed subject line in the federation protocol) has developed into a complicated and somewhat arcane cultural phenomenon of its own, with readers expected to be able to parse a line like ‘mh (-)’ to know what they’re being warned about (‘mental health, negative’). Basically... You know the kind of people who get sociopolitical religion and then want to ‘educate’ everyone else? Soi-disant marginalised people with elbows as sharp as their victim complexes? People who not only earnestly practise niceness but have set up a working group to determine niceness protocols? They tend to gravitate towards Mastodon. But then there are also people who really are somehow marginalised or just have a puppyish keenness, so, you know...
Oh, and one of the largest Pleroma/GNU Social sites really is overseen by someone who’s either a vocal racist or a gigantic edgelord.
Politically, it’s your usual social media drivel for the most part: loads of people banging on about late stage capitalism in a manner suggestive of late stage adolescence. There are a few people who are doing more serious thinking from a Leftist angle, but then again I just saw someone abandon an account on the social.coop instance, describing it as a bureaucratic hell... It’s a similar case for the Right: worthwhile conversations can be had, but some searching is usually involved.
So yes... You have been warned, and all that. In the event that curiosity burns yet within your heart (there must be some if you read this far, even if it’s maybe of a morbid sort), let me leave you with a few introductory following recommendations: @email@example.com (that entire instance has some great photographers, transcending all language barriers), @firstname.lastname@example.org (a strangely beguiling sort of dark fiction) and @email@example.com (everyone loves the kitten).