For Writers of Original Fiction

Well, more like "questions". Because I have two.

One: how would you write pm? Is is PM, P.M., p.m., or pm. Or do I just avoid using that in my writing?

Two: I'm not sure about hyphens ( - ). It's kind of hard to explain, but I'll try my best...

Here's a sample of a sentence from one of my short stories: I kept my bag in my lap. It kept all my belongings including two pairs of clothes–not including the ones I was wearing–and my wallet with my I.D. and the money I had made previously.

When I had my short story read by a beta they told me that I should have spaces between my hyphens, like so: I kept my beg in my lap. It kept all my belongings including two pairs of clothes - not including the ones I was wearing - and my wallet with my I.D. and the money I had made perviously.

My question is, what's the difference? I started doing that as I write, but when I read I see the the hyphens don't have any spaces in between? How do I know when to put spaces between the sentences and the hyphens?

Thanks to all in advanced!

~ J