While 「こんにちは」 [konnichiwa] is a formal greeting for "Good Day," to say "hi" people often simply say a sound like Oh or Ah, followed by the person's name, and an optional name suffix like "san, chan" or "kun." O, Mary-san! A, Ken-kun! The American everyday casual "hi" is not easily reproduced in Japanese. In general, using people's names is preferred when they are known. Avoid using the term "you" (anata) entirely [translates to "not self" and is interpreted rudely].