Look at the cards you already have and take inventory of them. It may be helpful to organize them using a chart. This should include the name of the card, current debt on that card, the card's limit, interest rate, how long the card has been open, and the yearly fee.
If you owe debt on your cards, come up with a plan to get it paid off. Focus on the high interest debt first and then work your way down. If you have multiple cards with debt, keeping paying the minimum balance to avoid late fees.
Once your cards are paid off, it’s time to come up with a plan to use your credit responsibly. You should always aim to utilize no more than 30% of your available credit at any given time. If you don’t have a plan to pay something off, you shouldn’t make the purchase until you do.