Asklemmy

Look at posts and comments from the instance admin. Does this person seem to know anything about running a server or managing software? Do you like the admin's tone and attitude? Does it seem like the instance will be around for the long-term, or is it someone's hobby that they may abandon on a whim?

Look at the instance's ban list. Do you agree with the choices? Maybe you want your All feed to include exploding-heads and lemmygrad, or maybe you don't.

Does the instance seem to have a community theme that you want to be associated with? Some emphasize LGBTQ+ communities, or NSFW content, or particular political views.

The transparency in funding and discussion of donations, the country or state it is hosted in (local laws), does its moderation policy align with your values?

If it's a small instance fewer communities may be federated already so you may need to be the one to manually search for them for the first time (thus adding them to your instance's "all" feed). Larger instances will already have many communities discovered and a fuller all feed. Though with lemmyverse.net and some determination you can build up the federated communities list up entirely yourself (and give everyone registered there an expanded "all").

I went on the "recommended instances" section here: https://github.com/maltfield/awesome-lemmy-instances

Ideological similarity.

Do the admins federate/unfederate from instances you care about? Do they ban/allow communities in a way you agree with?

Do you plan to create communities? If so, choose the instance where the communities best fit the theme of that instance.

If you just want the most popular and wide range of profiles, go with Lemmy.world