What exactly is the job that you've had? Have you learned things by having the position and gotten some enjoyment out of working in it? Also, why are you wanting to resign, please? If you're resigning due to personal reasons, it would be okay to put it that way.
However, if you're going to be writing a letter, then it would be helpful for us to know a bit about the position that you've had and other things about which I asked.
If you're resigning, please don't do so via an email. A formal letter that is mailed or hand-delivered will show more volition and integrity on your part. I would also think that you would like to leave the present job with the hopes that you will be able to use them for a recommendation for a position for which you might be applying in the future.