how can i get the balance of my itunes account

My account's balance shows at the top right of the store's page on my computer's iTunes, by clicking on my name towards the top right of iTunes, and by logging in to view my account via the Store > View Account menu option. On my iOS devices it shows at the bottom of, for example, the Featured tab in the App Store app under my account id (or you can view your account via Settings > iTunes & App Store and it should show on the screen that you are taken to)

If you don't see anything there your balance is probably zero.