Over the last week, I've been doing first-round judging for a writing contest which shall remain nameless. I'm bound by all sorts of confidentiality rules not to say anything specific about the entries, so I won't.
But from long experience as a judge, I know I'm not giving anything away by saying many entrants struggle with commas. They leave them out where they're absolutely necessary. They stick them where they have no business being.
So, if you know you struggle with commas or you get back a judged contest entry telling you so, check out this website. It covers most of the basics. Note especially Rule #4:
4. Commas set off non-restrictive (non-essential) clauses, phrases, and modifiers from the rest of the sentence.
That one comes up a lot when writers are introducing characters in their synopsis. Both of these are grammatically correct:
British sergeant Will Atkins is a brave soldier in Wellington's army.
Will Atkins, a sergeant in Wellington's army, meets Anna in June 1811.
But the following are WRONG WRONG WRONG:
British sergeant, Will Atkins is a brave soldier...
Will Atkins, a sergeant in Wellington's army meets Anna...
Think of this type of comma as a sort of mini-parenthesis: it MUST come in a set.
There. My inner English teacher feels much better now.