You are so happy as you have got a good job. After getting your appointment letter you must report to the authority on due date. This is called reporting. You joining must be documented. A format of joining letter is given here.
Human Resource Division,
[Name of the Company],
[Address of the Company]
Subject: Joining /Acceptance Letter
I am pleased to accept your offer and I have honor to inform you that I am joining the company from [Mention the date of joining] as a/an [Job Position/post name] in respect to your appointment letter dated [apointment letter issue date], Ref no. [reference number from the appointment letter). I understand and accept the conditions of employment that you explained in your appointment letter.
The position is ideally suited to my educational background and interests. I confidently feel that I can make a significant contribution to your organization, and I am grateful for the opportunity you have given me. I humbly request you to accept my joining/acceptance letter.