You know you have that idea and it's been settling in your head for years. Let's get to work on it. If it's a mystery, a list of things to remember. First, the plot needs what I call a middle plot, a major twist, and of course, the ending. It's not enough to just have the one idea that sets everything in motion. You can do it. Just plot it out in your head or on paper. Two: know everything about your character. I mean everything, right down to how much money is in your character's pocket. Likes, dislikes, favorite foods. Everything. You will not use all the information right away but you will need it. Think Sherlock Holmes. You know all his habits, right? Do the same thing for your character. Three: If your protagonist is flawed, that is perfectly okay, in fact, I suggest adding your flaws. Four, consider your setting. Will your book take place in a junk yard, or as one friend of mine does, her characters travel the world. Want to keep it home, use a great backdrop or setting. Trust me, setting is just as important as a character. Five: time. Yes, time. Please keep track of the dates as they go by. And if fact, when the tension ramps up, maybe the character is running out of time. Use time as your friend. Six: backstory and sub-characters. Your backstory for your character will help round out that character and the story. Seven: be very aware of crisp, good dialogue. Use it at will. Eight: Save a kick-butt twist near the end. This also refers to number one, but this is a must in a mystery. Nine: tease something for the next book. Leave out one possible plot point to be addressed in a later book. And finally, make sure the ending satisfies everything. And do not have a character at the end try to sum up everything in a long piece of dialogue. Now, go out there and write.