If you already have a different first page, as outlined here in this FAQ, the following may help you if you have a cover or title page on your document and don't want that first page to count as the first.
In the web/online version of Word, you will need to download a copy of it to edit in the client/desktop version, as the web version doesn't offer this. Learn how to download Office here if you don't already have the software.
Notice that the paper still starts at 2 after the running head in the screenshot below ("Different first page" has been selected). You will need to select the header area to get the options below to appear or go to Insert > Page Number > Format page number. 
In the popup that appears, select the Start at option and change the number to zero. The page will start the second page at 1 then:
