Is there an orientation manual for Lemmy?
Yes: read it right side up. Otherwise it's really hard.
More seriously, as to your question: my home feed is all (as opposed to just the communities I've subscribed). Unlike Reddit, Lemmy is still small enough, civilized enough, and patronized by people with decent enough mental faculties, that you can read all without being deluged with angry, stupid or flamebait messages.
Me, I only block communities that don't speak a language I understand - unless I have a compelling reason not to block them - and communities full of delusional or overly aggressive people (i.e. usually tankies).
I suggest you do that too. You'll get a very good feel of what goes on in here.