Steer clear from some instances, or at least, take everything they say with a massive grain of salt. Hexbear, Lemmygrad and lemmy.world come to mind.

If your client allows it, use the tag function to tag the same users from different instances, all the cross posts will become very obvious.

You might get contacted by Nicole, a bot that took a dark turn recently, just report and block.
Also report and block spam, sometimes it's porn, sometimes it's gore, other times it's ads, just report and block.

Don't be an asshole, and contribute by commenting and posting, or financially supporting Lemmy development or running your instance (if you can afford it obviously). Join smaller instances if you can, the more decentralized, the better. Also given the federated nature of Lemmy, you might see a whole different feed from one instance to the other, it's nice to have that option.