"Good luck" (two words) is something you say to wish someone good luck, it's like saying "I hope you have good luck". So you say it to someone who is about to do something difficult or something that is important.

The speaker wishes the recipient a positive outcome in some way. -I'm sitting my exam late today -good luck! (said in a voice that is smiling, wishing well) It can be used sarcastically - I really hope it happens (some unlikely event) - Good luck! (it would require good luck) (said in a voice that expresses scepticism) Or even more sarcastic -I have to find a house for less than... -good luck! (it is impossible) (said in a voice that is mocking, derisive)

It is two words: Good luck. I wouldn't say it was an idiom - as it is quite literal in its meaning. When you say it, you are telling someone that you hope they have good luck in what they are about to do. We often say it to someone when they are doing something new, or different, where luck might make a difference to the outcome. "Good luck with that exam tomorrow". You would say it to a friend who has told you that he is going to ask a girl out. Most people that sell you a lottery ticket would say it to you.

It would be, "good luck." It means, "I hope it goes well," or something similar, depending on the situation.