Mostly it doesn't matter for the person using it*, so you can just pick one that isn't overloaded to start. But...

Ways it does matter:

• Your instance's moderation policy and actions. (including what content is allowed/disallowed, how they deal with harassment, etc.)
• Server reliability. This can change drastically if a lot of people join at once, as many Lemmy sites have discovered this week! (I believe Lemmy.ml and Lemmy.world have both upgraded their hardware in the last few days to deal with this!)
• Admin reliability. This is harder to tell up front, but it's worth taking a quick look at whether the admins seem to be active and responsive, whether they seem like they're in it for the long haul or if they're experimenting, etc.

Switching is sort of easy in that all you have to do is create a new account somewhere, and you don't need to tell your followers because Lemmy doesn't have user subscriptions (though someone could follow you from, say, Mastodon)...

...but it's also not easy in that Lemmy doesn't have tools to export/import your subscriptions (yet?) so you have to add them to the new account manually. And moving your posting/comment history isn't something that's doable at the moment, either.

What I did when moving from lemmy.ml to lemmy.world was put the old/new accounts in each others' bios and add "Old Account" to the old one's display name. I'm not too attached to my post history sticking to my profile.

*I think it matters a bit more for where you set up a community, on the basis that an instance focused around, say, history would be a better place to create an archaeology community than one focused around FOSS. Though you might want to cross-post articles about free software used in archaeology!