Without the passcode, You will need to reset the device to factory settings unfortunately.. Keep in mind you will lose all data that is stored on the phone, so hopefully you have a backup! There is no way around this.
To reset this model you need to first power it off completely. Then hold the power button, home button, and volume up button simultaneously . Once you see the boot logo, you can let go of the power button. This will take you to the recovery screen. (may take a couple tries).
Then you'll want to use the volume buttons to select "factory data reset". It will erase the info, then reboot. Should be good to go from there.