Congratulations; you've thought of your plot. You've created your characters. Next hurdle? Whose voice is going to tell your story? Whose eyes will view the events? It may seem obvious, but wait; think about it. This decision affects everything else you will write.
Many times the easiest decision is to tell the plot from the viewpoint of your protagonist; after all, he/she is your main character for a reason. Writing in the first person brings an immediacy to your story-telling that is impossible to achieve with any other method. The writer can let the reader experience your protagonist's feelings and frustrations and see the world through their eyes. This can be an added bonus for mystery writers because if the main character makes a wrong assumption the odds are the reader will accept it as the right one. It's a great way to mis-direct. However, seeing the plot through only one pair of eyes can be limiting. Can you put your protagonist everywhere they need to be? Are they truly the one driving the plot? If your answers to these questions is 'no' then another voice and another pair of eyes are called for.
The omniscient narrator's eyes are very useful; since, being omniscient means they can see and know everything. This is when you can use the phrases 'little did he know' or 'it was the last peaceful morning'. It's nice to be the puppetmaster, but I find this style somewhat distant. I like to be in among the action not watching it as a member of the audience. That said, however, many, many wonderful books are written using this method.
But what about telling the story from a secondary character's point of view? Maybe it's not the detective telling the story, but the detective's trusty sidekick; or your main character's best friend. What about several people telling the story? Different voices add texture and depth. Several people can be in several locations which is a bonus if you're writing a mystery.
Of course, there are a myriad of more ways to write a story than I've written here, but I think every writer should think about whose eyes and voice best serves the tale they are going to tell. It might not be the main character. Take a moment to think about your current work; how would it change if it was told from another point of view? You might have picked the easiest voice, but is it the right one?