Formal:
* Bonjour, puis-je vous aider? (Good morning/afternoon, can I help you?)
* Bonjour, est-ce que je peux vous aider? (Good morning/afternoon, can I help you?)
Informal:
* Bonjour, besoin d'aide? (Hello, need help?)
* Salut, je peux t'aider? (Hi, can I help you?)
More specific to a store/shop:
* Bonjour, vous cherchez quelque chose en particulier? (Hello, are you looking for anything in particular?)
You can also use "Bienvenue" (Welcome) at the beginning of any of these phrases, especially in a shop setting.
Remember that the choice of phrase depends on the situation and your relationship with the person you are speaking to.