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.
Personally I like to place my business address at the bottom of the page or the footer. To do this go to the floating bar and click on the button, 'Switch Between Header and Footer.' This will open a dotted box at the bottom of the document. You can type your office address here or anything you see fit for your letterhead. Again you can format the text by choosing the font, font size, color, align the text, etc.