Before you write a word, you need to pick a protagonist. Who is your main character to be: a police detective, a private detective, or an amateur or from the academic world, the business world, or one of hundreds of fields? Look through the mystery section of any large bookstore and you will find detectives who are chefs, gardeners, musicians, park rangers, archaeologists, zoo veterinarians, lawyers, doctors, scientists—to name but a few. You might create a fictional character like you or someone you know. Best not to try to make this selection without knowing a bit about the field your character comes from,
The main character will probably dictate the tone of your book. Is your person funny or very serious, sarcastic or polite? What kind of tone will your mystery convey: hard-boiled, “soft”-boiled, or a combination of both.
Next, you need to pick the voice: first person, third person, or a combination of both. First person means that everything has to happen in front of your man character or be told about it. Third person can be more liberating in that you can take your readers into the lives of various characters as told by an omniscient narrator. If you combine both first and third person, be sure to make sure your readers are not confused. One way to accomplish this is to put the names of the characters in subheads.
Last of these preliminary steps is to figure out the other characters needed to move the story along. No one, especially an amateur sleuth, can solve the mystery that is the basis for the story, alone. This might mean a friendly cop or private detective, a trusted friend, or a law enforcement official in a government agency. At times, such characters might try to impede the work of your protagonist. Bad guys are key characters too, whether they be killers or obnoxious people your man person meets along the way. Nothing should go smoothly in a mystery novel. Conflict and barriers is what makes the book flow and keeps people wanting to read on.