I have activation lock on my MacBook Air. How to resolve this?

If you bought it and it has an activation lock. you need to take it back to where you bought it. Because if you don't know the key fro unlock it. You cannot.Activation Lock for Mac - Apple Support

The serial number is on the bottom of the laptop. You might need a magnifying glass to read it. You sales receipt should be good enough to return it. IF they are an authorized Apple distributer they may even be able to unlock it for you.