MYSTERY writing help?

Well, if you read my last journal entry- you know i've got writers block, and have to write a play.

Well, I decided to write a dun dududun!

MURDER MYSTERY && some comedy thrown in there. (THink Alfred Hitchcock 39 steps (sp?))

SO, any tips on writing mysterys? What kind of thing should i do? like innocent man accused, man sees murder and is like an eyewitness, or like Sherlock holmes where you're the detective?

I need to pitch an idea by monday, and need to do some outlining and charecter stuff- so if you could help my think of the basic thing that its about (like which of the things listed above or any other basic ideas like that)

I am so excited about this! The best play gets to be preformed by a local theater for friends and family, and could be our middle school play.

I'm more of a story girl, but having professional help for any writting is good for me :)