Hi,
Follow the instructions in this Apple support article > If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support
Regarding computer operating systems and iTunes ...
iTunes for Macs requires OS X v10.9.5 Mavericks or later and the latest version of iTunes > Get the latest version of iTunes - Apple Support
iTunes for Windows requires a PC running Windows 7 or later and the latest version of iTunes > iTunes - Download iTunes Now - Apple