I was searching some tips on internet and found the following:
Addressing American People
•Use last names with people you do not know: Address people using their title (Mr, Ms, Dr) and their last names.
•Always use "Ms" when addressing women: It is important to use "Ms" when addressing a woman. Only use "Mrs" when the woman has asked you to do so!
•Many Americans prefer first names: Americans often prefer using first names, even when dealing with people in very different positions. Americans will generally say, "Call me Tom." and then expect you to remain on a first name basis.
•Americans prefer informal: In general, Americans prefer informal greetings and using first names or nicknames when speaking with colleagues and acquaintances.
I hope it may be helpful in some way.