Jasmin,

Depends on the layout of your document and where you want the header.

If on the first page only then you can format using the Header and Footer
toolbar (use the little open book icon) to show a "different first page."
Once this is applied, you will see the header and footer lable on the first
page change.  Insert your header text.

If your page is buried inside the document, you will need to insert one or
more "section" breaks.  Headers and footers are a section attribute.  Each
section can have identical headers and footers, different first page, or
different odd and even pages.

Lets say you want a header on page 9 only of a twenty page document.  At the
end of page 8, insert a section break next page.  Display the header and
footer toolbar.  Put the cursor in the header of page nine (it will display
as Header - Section 2).  Toggle off "same as previous" (fourth icon from
right).  Using the format icon apply a different first page and type in your
header text.