Formal:
* "I am happy to assist you."
* "It is my pleasure to be of service."
* "I would be delighted to help."
* "I'm happy to be of assistance in any way I can."
Informal:
* "I'm happy to help!"
* "Let me know if you need anything."
* "Don't hesitate to ask if you have any questions."
* "I'm here if you need anything."
More specific:
* "I'm happy to answer any questions you may have."
* "I'm happy to provide you with more information."
* "I'm happy to walk you through the process."
You can also use variations of these phrases depending on the situation and your relationship with the person you are speaking to.