When to Cite
You should provide a citation whenever you quote, paraphrase, summarize or borrow ideas from the work of others, regardless of the source, media or format you borrow from.
How to Cite or Reference in My Text
The details of how you cite will vary by the discipline that you are writing in. But all styles require that you mark or set off the work of others from your own work in some prescribed way such as a footnote number or parenthetical reference at the end of your passage in which you use the material. That reference will point directly to a full bibliographic citation for your source at the end of your paper. That citation in turn will follow the format required by the particular style.
Three Pieces of Information You Must Have for Your Citations
All styles require the name of the author, the title of the work, and the publication or posting information. If you do not have all three pieces, then you need to re-evaluate your use of that source for academic work. Also, be aware that the publication information that is needed will depend on whether the source is a book, journal article or other format.
Author, Title, Publication for a Book
Author, Title, Publication for an Article
Author, Title, Publication for a Website