Familiarize yourself with what today's teens and tweens are reading. Start by perusing the magazine racks at your local bookstore, paying particular attention to the advice columns and the articles that are written by the target audience to whom you want your book to appeal. Gravitate to the YA section of the store, and purchase a few titles. Make note of who the publishers are. If you're not sure where to start, you can find plenty of titles and book reviews at teen-oriented websites such as Teens Read Too, N2 Arts, Young Adult Books Central and The YaYaYa's. If you have teens and tweens living under your roof, ask them what they're reading. Pay attention to the television shows and movies they like that may have characters in them who are their age.
Identify the age group you want to target. This decision will have bearing on the subject matter and genre of your book. For the 9-to-12-year-old age group, for example, many of them want to read stories with teenage characters, because they're already looking ahead to when they're going to become teenagers themselves. A lot of the plots for this age bracket involve events that are happening within the family unit, whereas books targeted to 13-to-18-year-olds focus more on the growing independence of the main characters, interactions with people outside the immediate family and the first stirrings of romance. These books often incorporate edgier themes than those for younger readers (i.e., school violence, substance abuse, peer pressure, teen pregnancy and suicide).
Decide on a plot to which your target audience can relate. Think back to some of the challenges you dealt with in your own youth. For example, if your family moved a lot, you can relate to the frustration of leaving behind old friends and trying to establish relationships at a new school. Maybe there was someone with whom you really wanted to go to the prom, but your No. 1 adversary was already moving in. Another scenario might be the pressure of being held to the high standard of an older sibling instead of being allowed to explore and develop special talents of your own. Yet another could be the annoyance of having to adjust to a parent's remarriage that introduced weird step-siblings with whom you were expected to get along.
Identify the core conflict in your plot. Conflict is predicated on reward, revenge, escape or a combination thereof, and is the driving force around which all events in your story will revolve. It's important to figure out what it is that your protagonist wants to accomplish, what risks he is willing to take to bring it about and what he stands to lose if he fails.
Decide on the genre of your book. Examples are comedy, drama, romance, fantasy, supernatural, mystery, science fiction and historical. This selection will then influence the setting and period for your YA novel. Experiment with this aspect of your plot before you start an outline. For instance, your YA readers might be able to relate to the contemporary backdrop of an urban high school, but shifting the same characters and dynamics to a mythical or futuristic setting might grab their attention more, because it's unusual.
Determine whether your story can best be told through the first-person perspective of the lead character or through the omnipresent third-person.
Decide how long your YA novel is going to be. The homework you did in Step 1 will give you a rough idea of what publishers expect. You can also look up the prospective publishers you'd like to court in "Writer's Market," an annual publication of Writers Digest books; the publishers include their submission requirements and desired word counts. Let's say your book is going to be 60,000 words long. Divide this by three, and each of the three acts (beginning/middle/end) of your novel will be 20,000 words long. (Once your writing gets under way, you can distill this further by deciding how many words to have in each chapter; this will give you a nice chapter quota to consistently shoot for each week.)
Create a working outline. List the events that will occur in Act 1. In Act 2, list the events that will escalate the conflict, and create more challenges and setbacks for your main character. Act 3 complicates the equation even more, puts the main character between a rock and a hard place, and forces him in the final chapters to take the biggest risk of all to finally achieve a resolution to the problem. Take note that endings to YA novels aren't always happily-ever-after, but they must suggest that the lead character has grown emotionally and/or spiritually from who she was at the beginning of the story.
Hit the ground running when you start your first chapter. Readers of any age, especially teenagers, have short attention spans. If something exciting, disturbing or dangerous hasn't happened to the lead character by the end of the first chapter, they're not going to stay around for Chapter 2.
Recruit teens and tweens to read your YA book in progress, and ask them for their feedback. Ask them if you've got the dialogue right, if the plot is exciting to them, and what they like and don't like about your characters. Many of them will be flattered you've asked their advice. Listen attentively, and respect what they have to say.