According to the APA,
A digital object identifier (DOI) is a unique alphanumeric string assigned by a registration agency (the International DOI Foundation) to identify content and provide a persistent link to its location on the Internet. The publisher assigns a DOI when an article is published and made available electronically.
. . . . The DOI is typically located on the first page of the electronic journal article, near the copyright notice. The DOI can also be found on the database landing page for the article.
Example of a typical DOI number: 10.1007/s10699-014-9361-3
The following online sites help you build a citation in a variety of formats, including APA.
(Note: Accuracy of citations created using the following online citation building tools may vary; always compare these tool-created citations with the print manuals for both the MLA and APA styles.)
Use the following practice exercises from Canada's Royal Roads University to test your skills in using APA style in a variety of areas: general skills (see the introductory video link on the first page under the "General APA exercise" link), formatting, in-text citations, and reference list:
The following documents will guide you in formatting papers in APA style (in both Word 2013 and 2016, as well as in Google Docs) with a different header on page 1 than throughout the rest of your paper, and with proper page number formatting. The first document bulleted below shows how to do so with screen shots taken from Word 2016, the second with screenshots taken from Word 2013, and the third using screenshots taken from Google Docs.
The fourth document walks you through how to divide your Word doc into sections, allowing you to use different headers and different page number formats within each (such as Roman numerals in one section and Arabic numerals in another, all contained in one Word file).
(Following these guidelines will save you a lot of headaches, allowing you to save your correctly-formatted paper in one single file.)