If I'm emailing someone I know, such as a teacher or coworker, I just write a quick hello and explain what I need before a quick thank you:
Hello Mr.Smith,
This is James Bond from MI6, I need you to submit the report on Superman by the end of today.
Thanks in advance,
James Bond
If you're writing to a person you just met, I prefer to use salutations instead of Hello.
Salutations Bruce Wayne,
My name's James Bond, or you can call me Agent 007. I am a superspy from MI6 and I have been to the Moon and back to stop villains. I will be working with you to take down Superman. Let me know a time and location so we can discuss the matter further.
Sincerely,
James Bond