If you are a beginner, I suggest that you do not try to do this, but to follow what LaTeX does by default (by the way, even I do this for all the letters that I write, and I have not lost anything).
If you are an advanced user of LaTeX, you can do a search on the following packages/style: isodoc, pbsi, and assign, and there are documents on the web. This will solve your problem. It will have many more advanced features like writing letters multiple recipients, etc. I have been meaning to create a Spoken Tutorial on this topic, but haven't got time to do it.