Tricked into getting a credit card

When paying for my items at the till, the cashier suggested I sign up for a card that will give me 20% off. I am a foreign national and was on holiday in the country when this happened. He did not explain that it was a credit card which I found very sneaky. I paid for the goods in cash and still received a bill for the items. I plan on moving to the country do I can't cancel the card because it will reflect badly on my credit. What a scam! I don't plan on ever shopping there again!