There are many great resources for debate available online. 
This page is just a sampling of those:

Middle School Public Debate Program (MSPDP)
MSPDP is designed to teach public speaking, critical thinking, listening, and debating to students in the middle grades. The MSPDP works with teachers, administrators, parents, students, and community members to form sustainable debating leagues and classroom oral literacy initiatives.
http://www.middleschooldebate.com

Debate Central at the University of Vermont
Debate Central has a large wealth of information on high school debate, including a large number of resources that can be accessed online.
http://debate.uvm.edu/

The International Debate Education Association (IDEA)
IDEA develops, organizes and promotes debate and debate-related activities in communities throughout the world.
http://www.idebate.org