こんいちは (pronounced as こんにちわ)
Hi, Hello, Good day
こんばんは (pronounced こんばんわ) Good evening
You will get along with these too in most situations. You can use them with your friends without being overly polite. Well, you can never be too polite in Japanese.
Still, there is a less formal greeting: どうも どうも means literally "very" and can be used as less formal Hi or Thanks.
Some more will come in two versions: a formal and an informal one:
|さようなら||Good bye. Note: Don't use this one when you're about to see the person on the same day.|
でわ（また）see you later
or you can just say まった
To say "see you next week" or "see you tomorrow", you can use また来週（らいしゅう）or また明日（あした）。