The definitive source for guidelines using APA style is the Publication Manual of the American Psychological Association, frequently referred to as the "APA Manual." Most recently in its 7th edition, the manual will contain information for formatting papers, in-text citation examples, bibliography help, and sample papers, including information specific to student papers.
The 6th edition is also still in use. Confused about which version to consult? Ask your instructor!
Both versions of the manual are available in the library, but be aware that most online sources for citation help, such as the OWL at Purdue, and citation generators like Bibme, Refworks, and built-in citation generators from websites will still use the 6th edition style for now. Be sure to check all citations generated by machine against the appropriate manual!
DOI is a digital object identifier
The DOI is a a web address linked to the publisher information of the article or book chapter. This is more stable than using a link from a database. A group of publishers, Publishers Linking International Association (PILA) formed the CrossRef depository as a collaborative linking system. To find more information go to https://www.apa.org/pubs/librarians/industry/crossref
To find a DOI for a publication you can use the CrossRef website: https://www.crossref.org/
A simple way to do so is to use the Search Metadata search and enter the title of the article. Then you can check for the matching item.