We continue to struggle with this word "so" -- so little and so much trouble!!
When "so" means "in order that" and implies the reason for doing something, it is a subordinate conjunction that begins a dependent clause, and there is no punctuation.
...walking slowly so I wouldn't miss anything...
...going to see her so I could give her the forms...
When "so" means "therefore," it starts a new sentence and takes a semicolon or a period in front of it.
...I had to leave early; so I missed the announcement...
...The doctor was in surgery; so he wasn't available...
The problem is that many people think "so" is like the word "and." But "so" is an adverb by nature. When it is pulled out to the front of the sentence and made into a conjunction, it behaves like the words "then" and "still" and "hence." It starts a new sentence.
There is no comma after it because it is one syllable.
Happy punctuating!
Margie