All disputes are handled directly with your Payment Gateway.
If you received a dispute or chargeback in PayPal, you will have to log into your PayPal account and follow their required procedures.
If you received a chargeback through Authorize.net, you should be receiving a letter in the mail that you will need to respond to.
If a chargeback has been a filed, a customer likely feels they have not received an adequate resolution to their order. This can be easily resolved by having a clear line of communication open to the customer and handling all inquiries promptly.