I want to host a murder mystery party.. but how???

So I want to host a Murder Mystery Party for Halloween but I don't know exactly what to do. It's for me and 7 other people. Does anyone know of any information I can use. I don't really want one of those murder mystery games in a box, I would like it to be basically free except for some supplies. Also I am only 13 and my friends area ll 12 or 13 so nothing to much adult rated but it can be gruesome.