You can either insert a section break and format the page layouts of the
sections differently or for a document (e.g. a letter head) in which the
presence of subsequent pages depend on content set the margin at .5"
throughout and set a different first page header. In the subsequent page
headers insert a borderless text box with wrap to push the text down to
produce the appearance of a 1" top margin.

I doubt you really want different margins, just different spacing. The only
way to get different margins is by using a section break. This can be
difficult to manage if you will be starting on page one and typing. You can
get different spacing easily by using a different first-page header and
using empty paragraphs in the continuation header to take up space. (To
change the left or right margin this way requires using a text box, too.)
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,