Write a summary or outline of your story, even if it is only one page in length. Because narrative voice is chosen to best fit the story, not at random, it is important to have some idea of what and who your story is about before establishing voice.
Choose whether you will use first, second or third person. First person uses "I" and puts the reader inside the mind of one character by telling the story from his perspective. With first person, the reader will only see the story through the eyes of that character, who may be unreliable as far as reporting events as they actually happened.
Third person uses "he" and "she" and tells the story from an outside perspective. It may still follow one unreliable narrator, or it may be a more reliable, factual telling of the story. Second person is used the least often of the three, and uses "you"; in this case, you must establish who "you" is that your character is narrating to.
Choose a tense for your story. The majority of stories are written either in present or past tense; either is fine so long as you are consistent throughout the story. Writing in past tense gives the reader the sense that the story has already happened and is unchangeable, while writing in present tense makes a story feel as if it is happening as the reader is reading it, and can add a sense of urgency.