Step 1 – Read, read, read YA novels! And I mean read tons of them until you are familiar with how they are written.
Step 2 – Know the language of teens. How is their language different from ours?
Step 3 – Teens take risks more so than adults. That either act on their emotions, or they let their fear rule their actions.
Step 4 – Teens are not perfect, no matter how much we want our own teens to be so. Teens are like kittens. They need to go off on their own and explore the world. So, therefore let your characters be willing to go away from all they know and discovery ups and downs of life and of the world before getting their happy ending. Or the ending could be where they learned from their mistakes.
Step 5 – It is critically important for your teen characters to change from the way they were at the beginning of the book. By the end they should have become a different person in some form or way. Physically, or emotionally.