Q: Where and how can i cancel a app on my iphone? I keep getting billed for it monthly.

There are instructions on this page for managing and stopping auto-renewing subscriptions : http://support.apple.com/kb/HT4098

e.g. if doing it on your phone then go to the bottom of the Featured tab in the App Store app, tap on your id, tap 'view apple id' on the popup and sign in, and on your account's page there should be subscriptions section.