That's not exactly how your credit score works. Both long term debt and revolving debt are bad for your credit score. And both are good for your credit score. You want to get a credit card or two to establish a credit history. A person with absolutely nothing on their credit report is a total wild card. Lenders want to see debt on your credit report so they can see how you manage the debt. But when they see someone who makes $30,000 a year with $30,000 of credit available to them that's a red flag. As I said, available credit is counted as debt in your debt to income ratio. They don't just consider it potential debt. They consider it debt, as though all of your cards were maxed out, even if you have always paid your cards off entirely every month.
I didn't say you should never sign up for a credit card, I said you should not be so quick to sign up for credit cards.