Title says it all. When I say it, I often pause after now. Not sure if it's correct to have the comma there or not.
A per def. 2 of Oxford the now you are using is to get attention to the rest of the sentence, in a conversational manner. Hence, both the examples in my link have a comma after the now.
Used, especially in conversation, to draw attention to a particular statement or point in a narrative:
now, my first impulse was to run away
I don’t like Scotch. Now, if it had been Irish Whiskey you’d offered me*