One more time…
The word so is normally an adverb.
…I was so very tired that night.
…He was so cranky at the end of the day.
It can also be a conjunction.
It can be the kind of conjunction that starts a dependent clause and is then a subordinate conjunction (like “because,” “since,” “as,” “before,” “unless.”) There are fifteen of these or so. When it is a subordinate conjunction, it means “so that” or “in order that” and always states the reason for doing something.
…I went to the doctor so I could find out the reason for the pain.
…She called so she could ask about the check.
…He went so he could have dinner with his brother.
When this clause is at the end of the sentence, it takes no punctuation.
Otherwise, as a conjunction, “so” means “therefore.” If you can substitute “therefore,” it is a conjunction that is called a “conjunctive adverb.” That means it is an adverb that has been pulled out to the beginning of a sentence to connect it to the sentence in front of it.
…She had resigned; so she was not there when the melee started.
…I have to be at work until 5:00; so I cannot attend the luncheon.
When it begins a sentence and means “therefore,” it starts a new sentence. The new sentence takes a period or semicolon in front of it.
The word “so” does not ever take a single separating comma in front of it.
And having said all of this, there are people who believe “so” can be like “and.” “And” connects equal things. “So” is just not the same kind of word.