Citing sources allows others to verify the used source and makes it clear who the author or creator of (part of) the used work is. At least the title, author, publisher, URL and date of retrieval (for a website) must be stated in this regard.
There are several style guides for citing your sources properly, for example the APA Style. You can also use software to save your references and include them in your project document. Some examples of this are Mendeley, Refworks and Endnote.