Most books have a third person narrative style, written as someone telling a story about others or as an observer of actual events. Third person narrative uses third person pronouns, such as he, she and they. Third person narratives have the choice of being omniscient or not, meaning that you can write from each character's personal viewpoint and thoughts, or you can write from just some of the character's viewpoints, or just one character's viewpoint. Writing from everyone's viewpoint may present challenges, as it can be difficult to maintain and consistently apply multiple viewpoints.
First person narratives mean that you are either writing as a character in the story or an observer to the story. Neither narrative position can have an omniscient viewpoint; you can only use the point of view you write from. The first person narrative provides an excellent way to focus on the point of view of one person, and works extremely well with stories of personal growth and revelation. An example of the first person narrative is Mark Twain's Huckleberry Finn.
For stories from a third person narrative, consistency can present a problem if you try to write from more than one person's point of view. Keeping a detailed list of individual character traits and other relevant information can help. Also, keep a timeline of the character's interactions.
For both first and third person, keeping a consistent chronology of events with clear markers for flash backs and flash forwards, if you use them, is a necessity. Also, if you write from the point of view of one character in a first person narrative, staying in character becomes important. You might want to develop a comprehensive biography about your character for your own reference as you write, including sample dialogue. But do not become overly concerned with details; it can stifle your writing.