Often people use Tables when they want to type text without having other things move around.

Another solution used with letterheads is to place fixed information in the first-page header and the information to be typed in the body of the document. This makes it marginally less likely that the letterhead will be messed up.

Take a look at the Tutorials on my downloads page. Another good resource on this is Setting Up Letter Templates.

Before you spend much time with Word, I would urge you to look at Basic Concepts of Microsoft Word - from Shauna Kelly and the pages that follow from it. Learn about Styles and Templates. The time you spend on this should be recovered in your first month if not the first week.