Who wrote the source? It might be one person, several people, or an organization. Also look for editors and translators, if they are credited.
What is the source? Look for a title. Articles, reports, good web-pages, and books usually have clearly identified titles. If it's an article in a journal or book, you'll need both the article title and the journal or book title. For any article, you will also need page numbers, if provided, and if you see one, the DOI (Document Object Identifier).
Where was the source published? If it's a book, who is the publisher and where is the publisher located? If it's an article, what is the name of the journal, magazine, or newspaper in which it appears? If it's a web-page, what organization published the website?
When was it published? If it's a book, what year? If it's an article in a quarterly journal, which quarter? If it's an article in a newspaper, what day? Many scholarly journals also use volume and issue numbers to further clarify when an item was published; you will need those. Websites sometimes only show a copyright year in the footer, but a more precise date is ideal.
The title page has the complete title, author(s), and publisher, but often lacks the date.
Publication Information Page:
The publication page has complete information, including the date, though the small print and many details make it harder to use.
Who: Margaret M. Willis and Juliet B. Schor
What: "Does Changing a Light Bulb Lead to Changing the World? Political Action and the Conscious Consumer"
Where: Annals of the American Academy of Political and Social Science pages 160-190
When: November 2012, Volume 644, Issue 1
You may need to check the table of contents page for full volume and issue information.
It's 2:00am. You have 15 tabs open, and you're taking notes. Wait... where was that tab with that one article about... wasn't it... no, it's gone. It's gone! Panic sets in. You can remember the general idea, and you need to include it, but the title, the author, the journal... gone. You can't even recall which database you found it in.
When you think you might use an article: that's when you record the information. Spark of interest? Right then. Looks relevant? Right then. Use a citation manager to make this step quick & easy.
Your list of sources with full bibliographic info might be many times longer than you'll end up using. So what? You can choose not to use them later. But if you don't record it now, finding it later will be... challenging.