Usually after you clear the interview a letter of confirmation is sent and you have to send an acceptance letter signed. This allows the employer to carry on with employment process and allow them making a contract. The acceptance letter should convey the message that you are willing to join the company and the tone should be courteous and show enthusiasm.
Thank you for sending me the formal job offer. I have read through it carefully and signed the letter, retaining one copy of the same with me as suggested.
I am excited to join ___________ (What is the Name of the Company) and thank you again for giving me an opportunity to join your team as directed on __________(What is the Joining Date).
I will do by best and contribute to the company from the day I join. I am looking forward to a long and productive career with your esteem organization.
Please call me if you need any additional information from me.
Name and Signature