When possible, avoid closing credit card accounts. The longer your credit history, the better your score. However, if you are very far behind in your payments, you may not have a choice. A payment plan may require you to cancel your credit card. If possible, though, keep your older accounts so that you have a substantial credit history on your side. (See also: How to Avoid Getting Your Credit Card Cancelled)
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.