There may be nothing you can do - oftentimes people who get hired later and are better at negotiating salaries will make more money. Also, do you and she do the same job? If you and she do the same job, at the same level of competence and the pay is truly inequitable, then have a talk with Human Resources about addressing this issue. Come prepared with evidence to back up your worth and why you deserve the same salary that she does.

Take into account the experience she brought into the job may cause her to be paid more than you. However, if she is earning a substantial amount more than you for doing EXACTLY the same job (not just similar), you should take it up with your HR department. You have every right to ask the question if she has been open about what she earns. However, be very calm about it as she may be being economical with the truth and trying to wind you up!