Discuss and learn French: French vocabulary, French grammar, French culture etc.
If you're wishing somebody "good luck" for something that will require 'mental stamina' such as exams, a long holiday with squawking children etc, then "bon courage!" would be a common way to say it.
For something that genuinely requires some kind of "luck", e.g. a lottery draw, then you'd probably say "bonne chance!".
Several years ago a french person told me it is more usual to say : Bon courage