/tmp/.s.PGSQL.532

Hello,
on $psql 
psql: could not connect to server: No such file or directory
          Is the server running locall and accepting
          connections on UNIX domain socket "/tmp/.s.PGSQL.532"?

Why do I get this answer?
What EXACTLY can I do to get this server running?
Please be so kind to give line by line what I should do. I am a novice again, and just want to exercise with psql.
with friendly greetings,
Peter 
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