Margin settings for your document can be found in your word processor's page setup settings. In Microsoft Word, this is found by clicking "File," "Page Setup" and "Margins."
When writing in APA format, the margin settings must also follow the specific guidelines. Set your top, bottom, left and right margins all to 1 inch. If your document will be bound, you can add 1/2 inch to the left margin to accomodate for the space.
If your primary stye of writing will be in the APA format, you can set Microsoft Word to use those margin settings by default. After you set your margins, click the "Default" button to make it the primary style. The next time you open Microsoft Word, your margins will be APA formatted.
In addition to correct margin settings, make sure to use the correct font, font size and line spacing to ensure the correct amount of information fits on each page.