On the top of MS Word you'll find the menu bar. Click on 'View' and then 'Header and Footer.' What appears is a dotted box already with a cursor inside. A floating menu bar also appears.
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.