Good night=おやすみなさい(oyasumi nasai). It's formal

But you can just simply say "おやすみ(oyasumi)". The informal way to say "good night"

~~~~~~~

"さようなら(Sayonara)"= educational books will tell you it means "good bye" but it's actually use as "farewell". When you will not meet that person for kind of a log time.

~~~~~~~

"じゃ、また(ja mata)"= so, see you. And "またね(mata ne) means also (see you"

~~~~~~~

がんばってください。^^

おやすみなさい/お休みなさい→Good night or Good night, (I'm going to bed) 

With friends, you can just say→おやすみ／お休み as well. 

さようなら→Good-bye (formal, can be used for a variety of situations and people)

じゃまた→ literally "Well, later/see you again!" （また　literally means again or also, so you are implying that you will talk to or see the person again soon)

However, じゃまたね or またね are much more common, and are said with friends. じゃまた後で and じゃあね are quite common as well.

So, just know that 〜　じゃまた後で

　　　　　　　　　　　　じゃまたね

　　　　　　　　　　　　じゃあね

　　　　　　　　　　　　またね

All have the same meaning! and じゃ／じゃあ is a bit optional, but allows you to politely 'switch topics' in the conversation, similar to "well, I gotta go" or "well, see you later" in English. It's all about the situation, who you are talking to, and what you want to express. Don't worry too much about this though! 

頑張ってください！

With Japanese, greetings are the least of your worries. Lol

To be as accurate as I think I am,

"Ja mata" is like saying "Well, later!"

"Mata ne" is like saying "Later, right?" I'd use that if you know for certain that you'll be seeing your friends later.

"Oyasuminasai" does mean "Goodnight". But it can be used as a way to say goodbye. I've seen it used that way in a formal setting, and only at night.

I also think that Oyasumi nasai means Good night.

When I learned similar expressions there was that meaning.

So what is right?Or this expression has two meanings??