When you write a book, you have an important decision to make right up front - what voice will you write it in? The voice is very important part of the book, and it can make or break your story.
What do I mean by voice? Voice is the style of how you write it. You can break it into three main parts: Mood, person, and tense. There are other factors, too, but those are the three main ones.
Mood is the emotion the book is supposed to carry. Some books have a cheerful mood. If your book thus, you would use lots of happy words and there would probably be some jokes. Descriptions would be full of color and sunshine. Other books have a gloomy mood. These books use sad words, and sad things happen. Descriptions tend to be grayish dark. Some have a hopeful mood, where there's grayish descriptions but there's a ray of sunshine piercing this darkness.
Person is what pronoun you use to describe your main character. Most books are written in third person. When a book is written in third person, your main character is him, or her. You refer to them by name. You are not limited to your main character, and can often get into everyone's head - or no one's, if you'd prefer.
The second most popular person is first person. If you write in first person, you refer to the main character as "I." This is the tense you write in if you're doing a "memoir" or a "diary/journal." Often, it is treated the same as third, except that you only get into one character's head. You have to make sure that your readers can figure out what the main character's name is (unless you purposely don't want your character to have a name ... I've read books like that) within the first chapter or so, or your reader will have trouble associating the character with the name. And I ask you, no, I beg you, do not, DO NOT change the P.O.V without warning. I have read books where, for the most part it is written in the P.O.V. of one character, except for a chapter or two that, without warning, are told in the P.O.V. of another. This is confusing, and it may take your reader a while before they realize that you have switched, especially the first time. If it is done consistently, and it is well marked that you have switched, and they have different writing styles and personalities, it can be done, but don't suddenly start doing it halfway through the book. You can also use the pronoun "We" if you're telling it from the point of view of two or more people. "The Magic School Bus" book series is a good example of that.
Second person is not used very much at all, for the main character would be "you" and this "you" would be your reader. It is mostly for instruction manuals and Choose Your Own Adventure Books. My mother says she has only read one book that was successfully written in second person that was not one of those.
The third part is Tense. There are three main tenses: past, present, and future. Most books are written in past tense. It tells events that have already happened. "Molly jumped over the crack in the sidewalk and went inside her house" for example. A few books are written present tense. It tells events that are happening. "Molly jumps over the crack in the sidewalk and goes inside her house." The future tense you will probably never see. My mom says she has only read one in it - a futuristic sci-fi - and that it was a very strange book. "Molly will jump over the crack in the sidewalk and will go inside her house."
How do you find the proper voice for your book? Experiment. Try writing it one way, and if that doesn't work, try another. I have some stories that I have tried to write in first person, but have ended up writing in third, other stories have been the other way around. Some authors write some methods better than they write others. My sister can't write first person, I can't write anything too extremely gloomy - a joke will always slip in. Find the style that works best for you and for your book.
That's all I have to say on that topic. If you have any requests for me to write about, leave a comment, and I'll see what I can do.