Research the types of titles you find interesting as a reader. Write the kind of stories that always keep you turning the pages.
Consider what kind of protagonist you want to feature. Choose from the hard-boiled side or someone cozy and genteel.
Think of an interesting hook to involve your readers in the mystery. Murder is not always necessary to raise a reader's interest.
Create a likable yet flawed protagonist, allowing room for him to demonstrate his human side. No one is particularly interested in reading about goody-two-shoes running around solving crimes.
Play with your detective's tone. The voice of your character can set the tone for the entire novel.
Consider what kind of mystery is best suited to the characters you have created. A trained detective may be a specialist in solving disappearances through uncanny deduction but a tough character may be better at throwing punches than working syllogisms.
Search the Internet to find some of the more successful detective fiction on the market. Look to see what houses publish them and whether or not they require agented submissions.
Decide whether you are going to shoot for publication with a major publisher or be satisfied with placing your fiction with a smaller house. Though small houses might lack the advertising budget of their New York counterparts, many place a high priority on establishing long and mutually beneficial relationships with their writers.
Research alternative publishing venues for your fiction if you find that the traditional route is prohibitive. Ebooks are an easy way to break into publishing while connecting you with a loyal and enthusiastic readership.