There are many online resources specifically for young writers. Here are just a few:

Tips For Young Writers From Published Authors and Editors

Elise Howard (editor in chief of Avon/Tempest Books for Teen Readers)
Sue Macy
Dian Curtis Regan
Aaron Shepherd
Elizabeth Winthrop
Maggie L. Wood
Elvira Woodruff

Other Resources

Poetry FAQ: A Young Poet's Guide To Poetry and Poetry Writing

My site. Includes advice from professional poets.

Tips For Young Writers

From a BBC article.

TeenLit.com

Network with other teen writers. This site publishes writing by teens, and also offers a writers' workshops, resources, and links.

Young Writers Clubhouse

Maintained by author Deborah Morris. Tips on getting started, writing FAQ. Offers an online critique group.

Also see these resource directories:

Google's Young Writers resource directory