If the credit card you have is a credit card with a high credit limit and you close it then closing that credit card could hurt your credit and lower your credit score.
It's best to keep credit cards open if possible even if you don't use them often.
Just use the credit card a little bit and pay off the balance owed and that will help your credit score rise instead of lowering it.
But if you really don't need that credit card anymore then canceling it won't hurt your credit too much but be aware that there can be a ding on your credit score that will stay on your credit report for at least a few years.
Also only open credit cards if you really need them and can pay them off because they too can hurt your credit.