I might have accidentally signed up for a credit card. What do I do?

i'm a dumb 18 year old kid. I was buying something at Sears (only $20 worth of things) and the guy asked me if I had a Searscard. I had their reward card (not credit) but he said they had "another card" that would give a discount. He never said it was a credit card, even when I asked him if it was, he blatantly lied and said no, it was just a discount card (and started rambling on about random things) and kept badgering me about it, until I finally agreed to sign up for it. Yeah I know, Im dumb but I entered my SSN and signed something... and then it didn't go through, he said. So he told me to enter my SSN again, which I did, and it didn't go through again. So I said "nevermind, I don't want to do this" and he finally let me escape, I paid in cash for my items and then left. Do you think I'm OK? Is there anyway I can call them and see if I was signed up? I just don't want a bad credit score or something :( I've never even had a credit card before, I don't have a job, or anything. Please don't tell me I'm stupid lol. I've always been told to never give out my ssn, but for some reason i trusted sears (never again). What should I do?