Resignation letters can be difficult to make, because you want to end the job you are currently working at, give them your notice (normally 2 weeks or more), thank them for the oportunity to work with them...and also leave the door open in the future (keep the possibility of returning open).
I would keep it simple and short and include the following.
How long you have been working there and the position
Give a reason (etc. Due to family responsabilityes I unfortunatly will be leaving my position as of___________ 2009).
Thank them for the opportuinity of working with them
End with something that is friendly and open. (I hope that circumstance might allow me to work for you again in the future).
That is a very basic and short resignation letter.
Take a look in google...just google "resignation letter samples" and you will get lots of options.
I hope that helps.
I look forward to seeing your finished letter.
All the best
- For Teachers