Ohayou gozaimasu. （おはようございます。） - Good morning.
Japanese people greet many ways depending on the time of day. Some greetings differ depending on the relationship you have with the person you are greeting.
One can also say just "Ohayou （おはよう）". This is casual, and should not be used with one's boss.
Konnichiwa. （こんにちは。） - Hello/Good afternoon.
There is a rule for writing hiragana "wa" and "ha." When "wa" is used as a particle, it is written in hiragana as "ha." "Konnichiwa" is now a fixed greeting. However, in the old days it was a part of sentence such as "Today is ~ (Konnichi wa ~)" and "wa" functioned as a particle. That's why it is still written in hiragana as "ha."
Konbanwa. （こんばんは。） - Good evening.
There is a rule for writing hiragana "wa" and "ha." When "wa" is used as a particle, it is written in hiragana as "ha." "Konbanwa" is now a fixed greeting. However, in the old days it was a part of sentence such as "Tonight is ~ (Konban wa ~)" and "wa" functioned as a particle. That's why it is still written in hiragana as "ha."
Oyasuminasai. （おやすみなさい。） - Good night.
Unlike English, "Oyasuninasai （おやすみなさい） is mostly used before going to bed.
Sayonara. （さよなら。）- Good-bye.
"Sayounara （さようなら)" can be also used instead of "sayonara （さよなら)." People do not say "sayonara （さよなら）" when leaving their own home. "Ittekimasu（いってきます）" is normally used. The response to "Ittekimasu （いってきます）" is "Itterasshai （いってらっしゃい）". "Dewa mata （ではまた）" is also often used as "see you later", similar to the English expression.
How about "See you tomorrow"? "Tomorrow" is "ashita （明日）" in Japanese. I think you can guess what the the answer is: "Mata ashita （また明日）".
I hope you can practice these greetings. I am going to tell you one more phrase, "Mata raishuu（また来週）". This means "See you next week".