I know there are a lot of writers who have a germ of an idea for a mystery novel. Most of them are mystery readers and feel like, if they can just get started, that they can finish an entire manuscript.
Here are some interesting and useful websites for mystery writers, and other writers, to help them get started on their mysterious voyage:
Write That Novel , which has useful, printable sheets for characterization, plotting, storyboards, etc.
Book Crossroads , which has links to online mystery writing groups, hardboiled slang dictionaries, forensic information, and legal overviews.
John Morgan Wilson's website , which gives tips on mystery writing, including a useful page that demonstrates how to bring your characters to life (if you've ever been told "show, don't tell," this would be good for you to check out.)
Holly Lisle's website , which has articles on creating characters, preventing a "sagging middle," ending your writer's block, etc.
A Yahoo Group for writers on firearms : a good place to start your research.
Tripod.com's Classic 12-Chapter Mystery Formula : Reading this can help you see the bare bones of many mystery novels. You don't have to follow it exactly--it's just a guide.
Advanced Fiction Writing.com's snowflake method of writing a novel : one of many different methods of writing/plotting a book.
The Cliche Site is just sort of fun. But it can also remind you how many cliches you have built into your writing. Take a look and see if there's another way to word some of the cliches that you've used.
Hope these help!