You will need the platform-tools from the Android SDK on your computer. Download the platform-tools as per your operating system.
Windows users will need proper drivers installed on their computer. You can try the simple FWUL adb/fastboot ISO or the Naked ADB drivers or the Universal ADB drivers if you don't already have a working driver installed
On your device, go into Settings → About and find the Build Number and tap on it 7 times to enable developer settings. Press back and go into Developer Options and enable USB debugging. From your computer, open a command prompt and type:
adb reboot bootloader
Open command prompt on your pc/laptop and run the following command:
fastboot oem unlock