Putting a header on one page only is especially easy if it's the first page. When you open the header or footer section, the "Design" tab that opens has an option called "Different First Page" located in the "Options" section of the ribbon menu. Click the check box beside it and enter the header for your first page in the usual way. This only adds the header (or footer) to the first page, leaving the rest of the document blank. You can add a different header for the rest of the pages or leave them blank.
If you want a header on one page only but not the first page, the process is a little more complicated. You do this using section breaks. You make the page in question its own section, and then it can have its own header. Add section breaks before and after the page to achieve this.
On your document, make sure your cursor is on the page before the one you want to have a unique header. Go to the "Page Layout" tab and under "Page Setup," click "Breaks." Under "Section Breaks," choose "Next Page." This sets up a section break at the end of the current page, so a new section starts on the next page. Navigate to the next page and do the same thing. Put a section break at the end of the page.
Use one of the methods for applying a header to the entire document to apply a header to the new section. It is automatically applied to the whole document, but you can change this. Select the header so you can type in it. In the "Navigation" section of the "Design" tab, you'll see that "Link to Previous" is highlighted. Click this to unhighlight it. This breaks the link between the header on the page and the previous header so that they can display different text. You may need to do this on the page after the one you want to have the header, too.
Use this method to set up multiple different headers in Word or to have a header on one page only. If you want a header for one page, you need to delete the header from the previous page – open the section and delete the text – and possibly from the page afterward as well.