How bad will cancelling my credit card hurt my credit score?

I was pressured into signing up for a Kohl's credit card yesterday (honestly I thought she was signing me up for some rewards or members card of some sort and I was in a rush so I just said fine to quit nagging me). I'd never signed up for a credit card before so I didn't quite realize what I was doing right away. Once I realized it was a credit card I thought about cancelling it. How bad will it hurt my score if I do? And do I run any risks of keeping it open? I read something about store credit cards being a mistake to sign up for with future consequences with higher interest rates for a future home or car... HALP!