People here say just restore...however, I talked to an Apple employee, and a DFU mode restore cleans out the system better than just a typical restore. I also would not recommend you restore from a backup.
To place your iDevice in DFU mode, hold your power and home button simultaneously while your iDevice is connected to your computer via a USB cord. When the screen shuts off, count from 0 to 5, then let go of the power button only (top right button), and hold your home button until your computer reads that it has an iDevice attached to it. Now, go and do a restore via iTunes.