Bonjour! literally means “good day” and is the most common French greeting. And the most useful. Why? You can use it in any situation, whether you meet with someone, enter a shop or a restaurant! If you want to be precise, you can say Bonsoir in the evening or at night!
There is also the word Salut, but be careful! It is more casual and you only use it with friends and family in informal situations!