Determine what kind of source it is. Is it a book chapter? A journal article? (Easiest way to do this is to look for a volume or issue number. Then you know it's a journal article!)
If it is a book, look in the catalog to see if we own it. If we don't, you need to Interlibrary Loan it. Do so by clicking "Check Availability" and then the "Request Item through Interlibrary Loan/E-Delivery" button. (This will require you to log in use your UD username and password.)
If it is an article, use the Journal List to search for the title of the journal to see which database it is in. (Journal titles are usually in italics in APA citations.) Use the rest of the information in the citation to locate it in the correct database. If the journal is indexed in a database, but we don't have access to the full-text, use Find It to request the title. If it's not in a database, use Interlibrary Loan to request the article. (Use the Article link under the New Requests menu on the left side.)