Chibby
Honestly, try out for inter-school debating competitions, even little ones. Talk to your debating teacher and let them know that you are interested. I did lots of DAV debates, made it to finals, and thus made it to the inter-state team. As for practice, learn how to write a debate under timed conditions. Also, practice for secret topics. This will not only prepare you for the actual debates, but it will be a great way of showing your capabilities and skills to get onto the team.
Is there a specific criteria for you to get onto the team?