Once you have your documents in Mendeley, you can cite and reference them within Word and OpenOffice by installing plug-ins. Once installed, you will have tool bar buttons that will allow you to cite a document, generate a bibliography, or manually edit any entry.
Go to Tools > Install... and select the plugin you wish to add.
For quick citations in most text editors, including Google Docs, select the paper you want to cite in Mendeley Desktop and click: Edit > Copy Citation. Then, paste it into the document you are composing.
Alternatively, you can drag and drop the paper from the Mendeley Desktop window to the document, and a reference in the current selected citation style will be added there.