Is it a sin to obtain a forged certificate of experience to work with, and what should be done in this case?

Issuing a fake certificate of experience is considered a lie, because lying is reporting something contrary to reality. A certificate of experience indicates that a subject has been studied for a specific period in a particular institution, and this certificate is not given to someone who has not previously taught. The questioner has made a mistake by falsifying it, and must repent to Allah. There is no blame on him for continuing to work as long as he performs it as required.