When you cite references to articles or books, you will need to put your citations into a specific style. This MSU Library Citing Resources Web site has information on some common styles used for citations.
A common style to use is the APA style.
When you cite sources I've listed for Finding Background Information (books in Stat!Ref, other electronic medical texts, Encyclopedia of Life Sciences or books from the library shelves), these will be cited as books or chapters of books. Note whether the chapter you're citing has its own authors. If so, cite the title and authors of the chapter as well as the title and editors of the whole book. If it was from an electronic source, you will also need to add in information about when you accessed the source. (Do not just cite Access Medicine but make sure you're citing the chapter title and books title within that database.) See guide to citing electronic resources)
When you get articles through searching the internet or a database like PubMed, cite the article, the journal title, the volume, date, and its authors. Do not cite the information as coming from "the internet" or "PubMed"! PubMed or Google are merely finding tools for the articles, but do not contain the information themselves.
You do not need to do anything special for citing online journals if they are respected medical journals indexed in a database like PubMed. These are unchanging and fixed documents, and so retrieval date is not important. If you are citing an article that you got off the internet from another source, you may need to cite the access date.
You need to pay careful attention to what type of electronic material you are citing. If it is an online book, follow instructions for citing books. If it is an online journal, follow instructions for citing journal articles. If it is a Web document that is neither of the above, follow general APA instructions for citing Web-based material.