Read popular young adult or teen books that are similar to the book you want to write. Researching the field and seeing what is published and what sells will give you ideas about format, length, style and content. Also look at classic books for teens to find consistent popular themes like alienation, identity crisis, puberty and social pressures.
Create a plot that is engaging and functions well as a story. To write a book that will appeal to teens, you must keep them interested and turning pages rather than turning on the television or using the computer. Make sure to have a strong central character and a good sense of conflict or action. Teens respond best to teenage main characters, but there can be some adult characters as well. Create a clear beginning, middle and end, with a clear resolution that closes the story successfully.
Write in simple, clear language. You do not have to use dumbed-down language, but you should avoid long, esoteric words or foreign words that may discourage teen readers. If you include original languages or words or scientific jargon, include a glossary at the end of the book to make it easier for the reader.
Give the book to teenagers and adults to read once you've finished. Make sure that both the teens and adults were able to understand the major plot points or ideas. The adults can point out inappropriate or troubling content for teens. Remember to tread lightly with vulgarity, violence and sexuality as teenagers may not be able to handle as much as adults.
Rewrite your manuscript once it has been read by multiple people. Let the book stay in a drawer or on your computer untouched for a while before revising. Approach the revision process in a simplified mindset and read your own words earnestly as a reader to see how you respond to the story.