Listen to what is hot on the charts right now. The music market determines what is in and what is out as far as influences in music.
Learn the audience. Search blogs, Twitter, Facebook and forums to understand the teen audience the song is intended for.
Focus on appealing to a particular area of teen interest, such as intimacy or partying.
Narrow the idea down into a workable concept. For instance, a song with an intimate theme could discuss feelings of worthlessness from rejection.
Write the song using a symbolic approach to lyricism. Don't state your intent directly; write metaphorically but in an unmistakable manner. For example, instead of "I feel all alone now that you're gone," try "My world's been so dark since you took your light away."
Write a chorus that is easy to sing along with. Use simple, one or two syllable words that are easy to remember, as this is the only part of the song that most people will know to sing along with. Capitalize on the theme of the song in the chorus. If the song is about loneliness, epitomize that feeling of despair in the chorus.