Saying "how are you doing?" is correct, but very informal, so if you're talking with somebody you don't know very well, you can simply say "how are you?". The answer to this question is very personal, depending on how you are feeling. "Ok" can be an answer, not very thorough, but always an answer. Then you have "Can anybody tell me what this song is about?" "present from Santa", without the article. "Those presents may be including..." "Do you believe in Santa?" or you can also say "Do you think Santa is real?" I hope this helped you, bye!