These links offer advice for using APA Style and have enough information for most student papers.
DOI stands for Digital Object Identifier. DOIs are used to identify electronic articles and function much like the ISBNs that you are used to for textbooks.
Unlike a web address, the DOI for an article never changes. APA Style recommends that you include a DOI when available.
Where do I find a DOI?
The DOI is often printed on the first page of journal articles--sometimes at the top of the page and sometimes at the bottom. You will recognize it, because it usually is preceded by doi: and the number always starts with 10.
Some articles will not have a DOI, especially non-scholarly articles or older articles.