The ideal is to create a letter template that takes the letterhead into account. Do NOT adjust margins to do this other than the margins you want for the letter in general. There is a tremendous amount of thought and work that goes into setting up a good letter template, whether it is to be used with pre-printed letterhead or printed entirely on the computer. It is lucky for you that most of that work was done some time ago and is available on the web for free.

See the letterhead tutorials Tutorials especially the one on using textboxes and styles.
See also Setting Up Letter Templates

In setting these up, you might want to import a scanned image of the letterhead into the first-page header, set to be behind text (not wrapping). You will remove this when you finish.

See also:
Templates in Microsoft Word
Understanding Styles in Microsoft Word
Basic concepts of Microsoft Word: An introduction by Shauna Kelly