Don’t worry too much about the instances, for the most part they are not that important. There are a few outliers tho:

• lemmy.ml, lemmygrad.ml and hexbear.net are politically in the Authoriarian Left extreme. Unless you either are politically close to them or like getting dunked on by people who think China and Russia are Heaven on Earth, don’t go there. Sadly the main devs for lemmy are situated on lemmy.ml.
• beehaw has a curated experience by blocking any instances which do not verify registrations (including the largest one at lemmy.world) and a different discussion culture than most. It’s a very chill place tho.
• and then you can also join instances that use PyLova (a fork of PieFed caused by … not cool actions by PieFeds dev, especially ironic since PieFed already exists partially because of lemmys roots on lemmy.ml), e.g. quokk.au. These give you a different interface and features, while giving you the same access to content.

There are also a bunch of outlaw instances, either far-right or tolerating CSAM. These you can only stumble upon if you actively search for them - they are defederated in a pretty coordinated effort, simply because due to the way lemmy works, their content would be replicated, creating a legal nightmare, and of course no respectable admin wants to touch that stuff.