Service Fees - Overcharged

I been a client with Standard Bank for awhile, a few months ago I app**** and received a credit card from Standard Bank. Initially my debit order from my Cheque account to my credit was not been done, I used to do EFT payment to ensure my account is paid on time. After visiting the bank and calling the credit card division, March 2016 was the first time a debit order was taken out after opening my credit card Jan 2016. But really shocked at the charges on the card, there's a service fee charge and a finance charge. When opening my credit card, my telephonic conversation with the consultant was that the only additional fee every month would be the service fee, which was understandable but an additional amount for 'finance charge'?. I called the call center again today and he did advise the finance charge is because I paid the minimum amount due and not the full balance of the card. From my understanding paying the minimum amount due is sufficient. Noticed my credit balance increased considerably. How can that be if I did not use my credit card in April? Really confused, should I have a finance charge???