I say there are three main things you might want to consider when choosing a mastodon instance, and these also apply to Lemmy.

The first is the native content of the server. You can follow anyone on any server (I’m not aware of any defederation drama over there like we recently had on Lemmy), but you do get a feed of local posts, which can be a good place to start when you are brand new.

The second is any local server rules. Instances can have rules around what types of content you can post (nsfw for example), and around content warnings (a lot of OG Mastodon users are really big on that). You should make sure you are happy with the rules before joining a server.

The last is the ongoing reliability of a server. I’d avoid joining anything really new or super small.

You can tryinstances.social. It asks you a few questions, and offers suggestions.

Choose an instance with a community that suits you. Like a hobby, interests or a regional one. I’m on a regional instance for northern germany, so my local timeline is all toots from that region which is pretty nice.

Don’t go for the big ones like mastodon .social, it’s against the idea of federation to have huge instances with millions of users