How do you know which formal prepositions to use? I mean how do you know when to say 'sir' or 'mister' etc 用 英語 (美國) 要怎麼說？

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This is a really good question because it's something native speakers do without thinking.
First of all, I have to correct you and tell you that words like "sir" and "mister" are not prepositions, they're forms of address.
People use these forms of address when they don't know your name. Sir and ma'am and miss are probably the most common ones in America. They're slightly formal, but they're used in both formal and informal speech. Things like "May I help you, sir?" and "Excuse me, miss, could you help me?"
"Mister" is also used, but it's a bit less formal and bit less friendly. For example, "Hey, mister, you can't go in there!"

In England, words like "love" and "pet" are used in the same way. For example, "Excuse me, love, would you mind stepping over here?"

Remember that all of these forms of address are used when you don't know a person's name.