Note that the American Medical Association (AMA) citation style is not preloaded into Zotero. To add the AMA style, follow these steps.
Open up Zotero, click on the 'Actions' button, and select 'Preferences'
Select the 'Cite' tab
Click on 'Styles'
Click on the 'Get Additional Styles' hyperlink
Search for AMA or the style you need:
Click 'Install' and the style will appear as a citation option in Zotero: