How to get my forgot restriction passcode?

Restore to a backup (if it was taken when the code wasn't on the device), or reset the device back to factory defaults (for both, first copy off anything that you want to keep) - to reset it back to factory defaults you may need to use recovery mode, there are instructions for doing that half-way down this page : If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support

(That link may not work, none of the Apple support pages currently work for me, hopefully it's only temporary.)