File > Page Setup "Lay out" tab > Check "Different first page" in the
"Headers and footers" section.

Now, set the top margin as you want it to be from page 2 onward.
Go back the the first page header, add your logo (or a bunch of ¶, or a
borderless autoshape, etc.) in order to push the top margin down on the first
page only.

In other words, the question should have been: "How do I set it up so that
the first page top margin is bigger than the rest of the document? !! ;-)

See http://sbarnhill.mvps.org/WordFAQs/Letterhead.htm for the answers to
this and related questions.