It is always safe to address people with common courtesy titles, such as "Herr Meyer" (Mr. Meyer) for any male over 20 years, "Frau Meyer" (Mrs. Meyer) for any female over 20 years (married or unmarried), and "Fraeulein Meyer" (Miss)for female adolescents between 14 and 18 years of age. First names can safely be used with kids and/or if your new acquaintance offers this. This brings us to the question whether you should use the address "Sie" (formal you) and "du" (informal you).
"How do you do?" in formal German would be:
Wie geht es IHNEN?
and in informal:
Wie geht es DIR?
"What do you plan to do today?" would be either:
Was haben SIE heute vor? (formal) OR
Was hast DU heute vor? (informal)
Now that is tricky for a foreigner, however you should at least know some of the rules which govern formal and informal speech:
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