Usually the first time you see someone in the day you greet them with "Good Morning," but if it's after 12 noon, it's more appropriate just to say "Hi" or "How's it going?"
"Good afternoon" is an acceptable greeting, but it isn't used in casual conversation like "good morning." It is more formal. (Unless you're being sarcastic while greeting someone who slept in or arrived late for work; then it's common to jokingly greet them with "good afternoon," even if it's before noon.)
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