おはようございます。= Good morning.
(おはよう is from はやい early. おはようございます is like "You are early.")
こんにちは。= Good afternoon./Hello. (Used in a daytime)
(こんにち = today, は is topic marker. こんにちは is a short of something like "How
are you TODAY?")
こんばんは。 = Good evening.
(こんばん = this evening/tonight, こんばんは is like "How are you TONIGHT?")
では、また。= See you again. (では = well then, また = again) じゃ、また。= See you again. (じゃ = casual of では) じゃあ(ね)。= See you. (more casual)
Q: (お)げんきですか。(お = polite) = How are you?
A: ありがとうございます。げんきです。= Thank you. I'm fine.
("(お)げんきですか" is not used often, because "こんにちは" and
"こんばんは" include "How are you?")
どうぞ。= Please. (Used when you offer something. ex. Here you are. Go ahead.)
すみません。= I'm sorry. Excuse me. Pardon?
ちょっとまってください。= Just a moment, please.
(ちょっと = a little, まってください = please wait)