How can I unlock my iphone as I have forgotten my original apple id

See if you can recover the ID/password. Use every email address you can think of.

Apple ID - How to find your Apple  -  An iCloud e-mail address is an Apple ID.

Apple ID - Find  -     

Other ways to find your Apple ID

You can check if you're signed in to the App Store, FaceTime, or Messages on your iPhone, iPad, iPod touch, or Mac, or check if you're signed in to iTunes for Windows on a PC. You can also go to iCloud.com or appleid.apple.com and see if your Apple ID prefills on the sign-in screen.   

If necessary, if you are the original owner, contact Apple. Apple can remove the ID  if provided the required paperwork to include the original purchase receipt with the serial number on it. 

Request additional support for unlocking your device.    -

You used to be able to take the device, the original purchase receipt with the serial number, and a photo ID to the nearest Apple Store (if it has reopened) and have them remove the ID. This may have changed, so call the store before going to make sure they can help.

Genius Bar Reservation and Apple Support Options - Apple.   -