안녕하세요? is correct. It is equivalent to How are you? in English.

"?" is correct. Usually we don't use "!" after "안녕하세요"

Thank you! ^^

I could say, either is possible. 안녕하세요? is used more generally, but 안녕하세요! can be used , too. if you use 안녕하세요! online, Nobody says that you are wrong and you should change the exclamation mark for question mark. you don't have to use that question mark. that is not important thing. :)