How do you say hi sir?

Answer 1

You speak the words, 'How do you do sir.' Then, if you are in the military greeting an officer, you salute; if you are not in the military or you are greeting someone not in the military, you extend your hand to shake his.

Answer 2

The way you have asked is quite informal and could be considered rude in some circles.

"Hello, Sir", would be much better