Payment history accounts for over one third of your credit score. So, while it may sound too simple to be true, the best thing you can do for your credit is to make payments on time. Every payment you make on time on any debt creates a positive item in your payment history. These payments stack up and offset missed payments that created negative items in the past.
However, there is a big myth that you have to borrow money and pay interest to get a good score. That is completely false! So long as you use your credit card (it can even be a small $1 charge) and then pay that statement balance in full, your score will benefit. You do not need to pay interest on a credit card to improve your score. Remember: your goal is to have as much positive information as possible, with very little negative information. That means you should be as focused on adding positive information to your credit report as you are at avoiding negative information.
This is easier said than done, but reducing the amount that you owe is going to be a far more satisfying achievement than improving your credit score. The first thing you need to do is stop using your credit cards. Use your credit report to make a list of all of your accounts and then go online or check recent statements to determine how much you owe on each account and what interest rate they are charging you. Come up with a payment plan that puts most of your available budget for debt payments towards the highest interest cards first, while maintaining minimum payments on your other accounts.