Make it up, explain where you are what job you are going to do, some family background, any sports/interests and that you will support the mess and look forward to seeing him on your arrival date. If you make an arrse of it you can always look forward to 'hanging off the bell'
As previous poster indicates, there's no fixed format for the content of a Reporting for Duty letter. I would suggest a letter to the CO, which will often be passed to your sub-unit OC for reply, and a letter to the RSM as you will be joining his Mess.
1. Intro. Quick intro identifying posting authority and reporting for duty date.
2. Your military background and experience. This will help your new Unit identify a particular role for you based on your background and qulifications. It is your chance to highlight strengths and influence what job you get (to a degree).
3. Your personal interests and any relevant family details, including whether you intend serving accompanied, living in SFA, what you want to do for relocation lve, etc
4. Conclusion. How much you are looking forward to particular aspects of your job / the Mess and pointing out any particular roles you think you might enjoy.
Send both letters early enough for the CoC to action anything that falls out of your letter - including sending a reply before you leave your last Unit. Keep the whole thing on one side of A4 paper (without using a microscopic font size).