How can I find my balance

See -> Check your Apple Account balance

Check your Apple Account balance – Apple Support (UK)

or 

Tap App Store -> tap your Profile (top right of the screen) -> then you will see your Apple Account balance.

See also -> Payment methods you can use with your Apple ID

Payment methods you can use with your Apple ID – Apple Support (UK)

fyi: Not all credit & debit cards seem to work with App Store / iTunes Store, try a different one, or preferably a credit card. If your card is not accepted as a paymenf method, you need to contact your bank.