I'm trying to create a header for a paper in APA, but I can't get the title to stay on left-hand side and the page number to stay on the right hand s
Here is the short answer.
a). Open your header and type your title on the left-hand side.
b). Use the tab key to move your cursor to the right margin where you want to insert your page number.
c). From your header tool bar, click on the page number icon and use the option to insert the page number in the “current position.”
This should do the trick.
For more detailed information on format papers in Word for APA, see this Library page - http://libguides.css.edu/content.php?pid=61826&sid=454618
2017 update: Microsoft Word offers a pre-formatted template for APA papers. Look for it in the right hand column of this page.