Your first thought was probably to ask for a raise. Understandable. But make sure you go in armed with the right information.
Did you negotiate salary when you were hired? If you didn't and your coworker did, it's no wonder he/she makes more money. But before you go in and try to remedy that, do some research. Head to Salary.com's free Salary Wizard and figure out what the median salary is for your job in your specific location. And if you want an even more precise salary range as well as other helpful negotiation tools, consider buying a Personal Salary Report.
After all, you might find out you're already paid fair market value for your experience and education level. But either way, you should have a target and a plan before you go in for the negotiation.