This will give you an idea of the type of book currently selling. If it's been a while since you've been around 12-18 year olds, observe them in their natural habitat. Even if you're writing a historical set in Roman times, the behavior you see today is not THAT different. Remind yourself of what puberty does to your complexion, not to mention other body parts. Decide on the age group you want to write for.
Opinions vary as to the age group for young adults.
If you write for the 12-18 age group, your novel will max out at approximately 60,000 words. Decide the genre you wish to write and focus on successful books in that genre. If you want to start a series, read The Hardy Boys or Nancy Drew to see how the author sustained interest through book after book. Now that the research phase is done to your satisfaction, come up with a unique plot line. To find out what young adults are interested in, visit places on-line where teenagers hang out.