I have a 12k credit card balance at what will be 15% interest what will be my monthly responsiblility?

Answer 1

When a retailer puts a late fee of 1.5% per month - you multiply that by 12 months to find your annual percentage rate. That reatailer is charging 18% per annum.
If you have an annual percentage rate of 15% on $12K the fee is $1800 per year divided by 12 months = $150/mo just for the finance charge.

Answer 2

Nothing, if you don't charge anything on it.

Look at the paperwork they gave you - it should say what the minimum payment will be. But if you only pay the minimum payment you'll be paying a lot more than you realize for the items you buy.