Make sure to take this with you when you go to your boss. Know what other men and women in your field are paid. Focus on your contributions to the company. While the raise is certainly important to you, do not focus on how it will help your credit card debt. Know the company's policy on raises by asking your human resources director.
Timing is everything.
Don't ask when the office is hectic, wait until the pace has slowed down and the moment is right. Do your homework. Rehearse and prepare responses to counter any objections your boss might have. If you can, act out the scenario with a friend or colleague. This will help you to become more comfortable when you are actually doing the asking.