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The exact conventions vary per country. But generally, the closer you are to a person, the more you're likely to use the familiar form "tú". Here are some examples that work in most cases:
Complete stranger: usted
Your friend of the same gender: tú
Your friend of the opposite gender: varies per person and sometimes age
Your acquaintance of the opposite gender: usted
Your acquaintance of the same gender: varies per person and sometimes age.
Your significant other: tú
Your mother: tú