Aside from the points made so far:

Expect it to be overwhelmingly very left-wing and radically progressive on social values.

You will get downvoted for expressing opposing or even skeptical opinions, often brutally. It doesn't really matter, Lemmy's 'Karma' system doesn't work like Reddit's does, but just expect that.

My account is like 12 hours old but I was trying out Lemmy on other servers well over a year ago. Nothing about that has changed.

You might feel similarly to the majority and therefore you'll have no problem.

If you don't, eh, well. There we both are.

Also, yeah, post links and create subreddits about stuff you care about. These sorts of alternatives only live if people engage with them.