When you're done, you can click on the 'Close' tab on the floating bar. The items in the header and footer will appear greyed out, but don't worry; this only means the they have been set. The cursor will then appear on the first line where you can begin typing your letter.
You also have several templates to choose from with Open Office software, but Microsoft Word provides a wider selection. However, if you do find a template that seems good enough and you'd like to customize it, go to Wordlabel.com to download the file. To open it, right click Open With and then select the software. You can then modify it however you want.