Make sure you have this code on the bottom of your page:
At the point in the text where you want to insert the citation, add this code :
<ref>text of your citation</ref>
Or use the "Cite" on your toolbar for templates to help you.
Edit - Cite gives you 3 options, automatic (created by using the url or isbn), manual, or re-use (an existing citation).
The manual option offers templates for common types of citations, such as books, journals, news, and websites. However the default is for very simple options, like a single author for a whole book.
What if you need more fields?
Such as for multiple authors, editors, chapters of books and more? Scroll down to the end of the template for a given type (example book), and click on Add more information.
You can then search for the field you need, or scroll until you see the option you need. Still can't find what you need? Follow the additional information link at the top of the template for more examples with the wiki code you can insert when you use Edit Source.