Just opened my first credit card

So I just opened my new first credit card, and I’m reading the pamphlet they sent but I’m a little confused. My mom said that whenever I purchase things using my credit card, even if I pay the full balance every month I’ll still be paying more than I used the credit card for. Like if I bought $30 in gas I’d still pay the $30 plus interest on that, even if I pay it on time. I’m having trouble finding this info. By the way, the card is through capital one, the card is called capital one platinum and the APR is 24.99 if that helps.