Learn as much as you can about your characters before you start writing and revisit your notes all throughout the writing process. You may find later on that a character looks better with green eyes instead of blue. Learn about their history, their families and what conflicts they might have going on in their lives. These existing conflicts will help shape your characters.
Build your world by picking your setting, or time and place where most of the events in your novel will occur. For example, if you're writing for the young adult audience, your main setting might be with the kids at school. Outline your novel with details such as which characters have classes with one another and who spends lunch together.
Introduce your characters and their relationships to one another as early in the book as possible. This will allow your readers to know all they can before jumping into your world. This will also give your readers a chance to pick their favorite character or characters.
Add conflict to your plot such as man vs. society, man vs. nature and man vs. himself. Keep in mind that you can use a number of conflicts within a novel, so you aren't limited to just one. It's common for conflicts to overlap one another as well. Detail the events while working your way toward the story's climax or ending.
Tie up any lose ends at the end of your book to give your readers a sense of accomplishment. Even if this novel is the first in a series, most plot lines will finish in the first book to open up for new plots and characters in the second one.