I got pre approved line of credit and have a few questions

I got pre approved 7,500 line of credit form scotia bank randomly on the app and opened the account and have a card being mailed to me. Can someone explain how minimum payments work to me please and how It works if you are in debt on the line of credit for example 500 I heard they can randomly request it right away? What if the person doesn't have it right away that's the whole point