So is a very common discourse marker in speaking. It usually occurs at the beginning of clauses and we use it when we are summarising what has just been said, or when we are changing topic:
[from a lecture on English literature]
So, we’ve covered the nineteenth century and we’re now going to look at all the experiments in the novel in the early twentieth century.
[discussing whether to eat a pudding or keep it till the following morning]
A:
I’m not having it cold in the morning.
B:
Oh. So what sort of pudding is it?
So, what time does the film start?