Firearms for Fiction Writers

You are a writer and you want to bring some good conflict to your story, but a hammer versus knife battle just does not fit in your story, and you don't know the first thing about guns. So how do you write your story and not make major mistakes that some of your readers will pick up on? (This information could also be useful to anti-gun activists who are trying to ban various weapons or accessories but are ignorant of what they are talking about.)
I will try to present most of what you need to know here; types of firearms, loading, firing, casualties, and tactics.