If you’ve tried making a purchase but your order was cancelled, you will not be charged for that order.
However, Google Play may put a temporary “reservation” on your credit card that's equal to the value of the purchase. But don’t sweat it, these “reservations” clear automatically in a short period of time.
Canceled purchases are likely to occur if your billing address, and/or the name registered in your Google Wallet is different than the one on your credit card. Straighten this out by contacting your bank to confirm details, then log into your Google Payments (at payments.google.com), and enter “Payment Methods” to verify that your information is consistent.