In Provence, where life is lived determinedly in the slow lane, nothing is more important that stopping to say bonjour. If you only speak one word of French, make it bonjour. Nothing is more certain to put your waiter or sales assistant in a grump than not saying hello as you enter their domain. Taking time to smile and say bonjour, even if you have nothing to follow it up with, will ensure decent service.
After about 6pm bonsoir will replace bonjour, and though au revoir is a catch-all for goodbye, you may hear bonne journée (have a good day), bonne après-midi (have a good afternoon) or bonne soirée (have a good evening).
A bientôt! (See you soon!)