Make a list of the things that stressed you out the most when you were between the ages of 12 and 18. Examples: getting good grades, being popular, having a crush, moving to a new town, adjusting to a divorce/remarriage, earning money. Not surprisingly, these same themes were probably present in your parents' and grandparents' teen years as well as the current generation of young people. Choose the one that resonates the most strongly and jot down as much as you can recall about the emotions, experiences, risks and rewards associated with that chapter of your life.
Create a young protagonist to place in a similar circumstance. YA heroes and heroines should be the same age as your target readership as well as the primary voice/presence throughout the story. Identify the protagonist's quest and what is at stake if she fails. List the talents and skills she possesses to achieve her goal. Create a second list that reflects the external and internal obstacles standing in her way. An external obstacle can be another person, a lack of resources or a race against time. An internal obstacle is a fear she needs to overcome in order to be successful.
Determine the time-period and setting for your YA novel. While teens and tweens relate well to stories set in environments with which they're familiar--school, home, neighborhoods--a good theme can transcend these parameters and unfold in the Dark Ages, the Old West or even a futuristic planet. If you love history, YA fiction is an effective way to gently weave in facts as well as have fictional characters interact with individuals who really lived.
Develop an outline before you start writing. The word count for YA fiction ranges from 40,000 words to 75,000. Whatever length you're aiming for, divide the total by three. This will tell you how many words to allocate to the beginning, middle and end of your novel. The beginning needs to introduce all the key players and the core conflict. The middle escalates the conflict and causes the protagonist to doubt her abilities. The ending resolves the problem and ties up all of the loose ends.
Experiment with first-person and third-person narrative to see which one best delivers your story. Eschew passive voice, keep your chapters short and strive to end each one with an exciting cliffhanger that dares the reader to turn the page. While it's acceptable to have adults in your novel, they should neither be the problem solvers nor the ones who steal the spotlight from their young counterparts.