1. German speakers say "Hello!" in a number of ways.
This partly depends on the time of day:
"Guten Morgen!" is said until about 10 a.m.
"Guten Tag!" is said from about 10 a.m. to 5 or 6 p.m.
(There is no equivalent phrase for "Good afternoon!" in German.)
"Guten Abend!" is said after 5 p.m.
All of these greetings are used in formal situations when we might say "How do you do" in English, or when meeting people we don't know particularly well. When you expect or are hoping for service, it could be interpreted as impolite not to say "Guten Tag!" as an opener.
2. In informal situations however - amongst family, friends or young people -, the above greetings are often shortened or other greetings are used:
