Not able to add Card billing info.

Ok, so I had my Billing information all filled out and I bought an item in game on my iPad. I tested it out but i didn't like it, so then i requested a refund. Later i tried two other in game purchases but neither were good enough, so I regretted my purchase and refunded them. So this is where the problems begin. So later in the day I wanted to download a free app, but then it said that i needed to update my billing information, then it tells me that i can't use my card anymore. I check all my purchases and it says that i owe Apple a bit of money, apparently my account didn't have enough money on it at the time, what do I do? I don't have another card and it won't let me add the card again.

Thanks for your answer.