In order to switch from EndNote to Zotero, you will need to export your EndNote library as XML file type, then import it into your Zotero library. If your Zotero library has existing collections, it is recommended that you make a backup of your Zotero data directory (instructions for how to do this can be found here).
The steps to export EndNote and import in Zotero can be found on the Zotero website here.