Most English speakers who want to say "good night" in Spanish will end up saying "buenas noches". This is the literal translation of "good night" into Spanish. However, "buenas noches" is commonly used when you first run into people at night, much like "good evening" is used in English. Keep this in mind. But also keep in mind that it is fine to say "buenas noches" for "good night".
If you are going to see the person or people the next morning, then it may be more appropriate to say "hasta mañana". This means "until tomorrow" and is commonly used when you are saying good night to someone who you plan to see the next day.
There are a host of other ways of saying "goodbye" to someone. All of these ways can be used to say "goodbye" at the end of the night as well. Click on the resource link below to read more about how to say goodbye in Spanish.
