When you find an article online, you can often find all the citation information you need on the article's landing page. If you search for an article in a database, you can use the citation elements from the search results. Remember to always verify the database's citation with the information from the full-text or PDF of the article!
You can often find all the information you need for a journal article citation on the first page of that article. Occasionally, you may have to look at the title page of the journal to determine the date or volume number of that particular issue.
When citing a book, you'll usually find the information you need on the title page and the back of the title page (title page verso).