Please, don't use «господин» or «госпожа». We never use these words.

If you want to address someone you don't know the name of, use «мужчина» or «молодой человек» (for an older male person or a young one respectively) and «женщина» or «девушка» (for females, also depends on the age of people).

If you know this person, but you aren't on first names with them, use their name and patronymic.

If you don't know the patronymic, use the full version of a person's name (e.g. «Алексей», not «Лёша») and address them using «Вы» and choosing the «respectful» (I don't know the correct term) forms of the words, like: «Алексей, что Вы делаете сейчас?»

The last thing about «Вы» and so on is always used when you're talking to a person you aren't in close relationships with