The rules for “so”:
When it means “so that” and implies the reason for doing something, it begins a dependent clause which, at the end of the sentence, gets no punctuation.
…I sent it to her so I could get her opinion on the content.
…He called so he could verify the information.
When it means “therefore,” it starts a new sentence and gets a semicolon or period in front of it.
…I didn’t know; so I couldn’t really do anything about it.
…She was underage; so there was no question of whether she was allowed into the event.
I know that there are people that support the idea that “so” can be just like “and” and “but.” I don’t think it is the same kind of word. “So” is really an adverb. And when an adverb is pulled out in front of a sentence, it becomes a conjunction and is called a “conjunctive adverb.” A conjunctive adverbs begins a brand-new sentence and needs a semicolon or a period in front of it.