How do I get the system lock pin code?

If you are seeing this screen:

you are being asked for a code to log in to your computer. This sounds like you have your computer set up on Find My. If you do not remember the passcode for this, you can turn off the Activation Lock from your iCloud account:

1. Sign in to iCloud.com with your Apple ID.
2. Go to Find My iPhone
3. Select your Mac from the Devices menu. 
4. Select Unlock. After you follow the steps to verify your identity, you'll see the passcode that you need to unlock your Mac.

Are you referring to the following screen?

If so, it is asking you for the firmware password for this notebook which was enabled sometime in the past. You would just need to enter that password to regain access to the notebook. If you forgot that password, you will need to make a Genius Bar appointment at your local Apple Store or Apple Authorized Service Provider for further assistance. Note that you will need to bring a copy of your original sales receipt or invoice as proof of purchase.