Paying back the full balance each month is the best thing you can do!
It's the proper use of credit cards.
However, since you don't have much of a loan, it has little effect on your credit score. The biggest effect is when you get the credit card in the first place Getting a credit card does lower your score, but using the card wisely soon brings it back up again. And, if your credit card balance goes up towards its maximum and you don't pay it off relatively quickly, it has a negative effect on your rating.
Generally, credit cards don't have much of an effect (even when you don't pay them off in full, but keep the balance low) on your credit rating.
Oh, if you want to boost your rating, then look at taking out a meaningful loan and pay it off on time - more effective than using a credit card.