Set uniform margins of at least 1 in. (2.54 cm) on the top, bottom, left, and right of every page. Use your word-processing software to add a header that will appear at the top of every page that includes the running head and the page number. The header appears within the top margin, not below it.
According to the APA Publication Manual 6th edition (section 8.03, pp. 229-230).
For more information about editing and formatting of essays, manuscripts, and theses, see the Owl Editing website.