Hi=やぁ！こんにちは。it's a casual greeting. You can say "hi" anytime in English, but in Japanese, it depends on when you say it. In the morning, it changes おはよう(morning!), In the evening, こんばんは. Hello=こんにちは (good afternoon). formal way to say. In the morning, おはようございます(good morning), in the evening, こんばんは (good evening). Don't get wrong ways to use. There are 3 ways for greeting in Japanese. Not like English "hi" and "hello".

１；やぁ！(common word) ２；どうも！common word) ３；やっほー！(sounds a friendlier/ super casual/a little girly)

ya :) やあ :)

ｳｨｯｽ～(ﾉ∀`)

@Kristie ありがとごさいます　:D