EndNote is designed to be used with MS Word and this is the most straight forward option. EndNote can be used with Pages by downloading the Pages EndNote Plug-in found here. EndNote can be made to work with Google Docs using a drag and drop or copy and paste method but the citations will be unformatted and the document will need to be downloaded in rich text format (RTF) and formatted using EndNote. Here is a video on how to do that.
When you download EndNote to a computer, it automatically installs an EndNote toolbar in Word. This program is called Cite While you Write (CWYW). The location of this toolbar can vary, usually depending on the version of Word you are using.
For PCs, it is usually a separate tab near the top of Word:
For Macs, it is usually located under Tools, EndNote
From this toolbar, you can search your EndNote library by author, title, or keyword to find citations, insert the selected citations in-text (which will also automatically create endnotes or a bibliography, depending on the style), change styles, and edit the in-text citations.
Please note that the information that appears in the in-text citations and at the end of your document is dictated by your selected style.
If you have installed EndNote and CANNOT SEE your toolbar, please see EndNote's FAQs on this topic.