A DOI, or Digital Object Identifier, is a string of numbers, letters and symbols used to permanently identify an electronic article or book and link to it on the web. The DOI helps readers locate a document even if the physical location of the document changes. The DOI will never change.
The DOI is typically displayed prominently in the bibliographic information for a publication. Check the first page or first several pages of the document, near the copyright notice. The DOI can also be found on the database landing page for the source.
If you still can't find the DOI, you can look it up on the website CrossRef.org (use the "Search Metadata" option).
It's important to note that not all electronic materials will have a DOI. The DOI system is a fairly recent concept, so books and articles published prior to 2000 are less likely to have DOIs.