APA uses an author-date citation format (ie. Smith, 2010)
Regardless of the resource you are citing, you will likely need an author, date, title, source, and specific format information (such as a volume number, page numbers, a URL, publisher, etc.).
The information required for your citation will depend on what you are citing (book, journal article, report, etc.) and the format (print, online).
What is a DOI and when do I need one? This APA flowchart walks you through the steps of finding a DOI, and when to use the journal or publisher home page URL instead. CrossRef offers a DOI look-up tool. Copy and paste portions of your citation and it will find the DOI if available.