Take a look at: How to set up letterhead or some other document where you
want one header on the first page and a different header on other pages.
http://www.addbalance.com/word/headersfooters.htm This gives step-by-step
instructions. (It also has the following links)

Some other pages to look at:

Letterhead Tips and Instructions
http://home.earthlink.net/~wordfaqs/Letterhead.htm

Letterhead Textboxes and Styles tutorial
http://addbalance.com/word/download.htm#LetterheadTextboxesAndStylesTutorial

Template Basics
http://www.addbalance.com/usersguide/templates.htm

How to Create a Template - Part 2 - essential reading
http://www.mvps.org/word/FAQs/Customization/CreateATemplatePart2.htm

Word "Forms"
http://www.addbalance.com/word/wordwebresources.htm#Forms and

Word for Word Perfect Users
http://www.addbalance.com/word/wordperfect.htm if you are coming from a WP
environment (or even if you are not).

If you are interested in creating templates that will work with the letter
wizard or use that wizard, you should look at the chapter on Advanced
Document Formatting in Using Office 2003 (or whatever your version is),
Special Edition, by Ed Bott and Woody Leonhard. It has detailed instructions
including instructions on getting the fields you want from your Outlook
Contacts for addressing a letter. (Chapter 19 of SE Using Office 2003) You
should be able to get this through your public library or at Amazon.com
http://www.amazon.com/exec/obidos/ISBN=0789729555/balancecheckbookA/

Finally, take a look at the letter templates that come with Word. While they
are no great shakes as letterhead, they do use styles and AutoText lists
very well. If you use the same style names that are used in those templates
in your own letterhead for the same parts of the document, you will have
better luck with using the built-in AutoText entries in Word.

Hope this helps,