Using the Zotero app on iOS devices is most effective. After downloading the app, you can share sources located in a browser app using the "share" function. Depending on your browser, this may look a little different or be in a larger set of tools. Look for the icon that looks like a box with an arrow pointing upwards. Here is an example of what that looks like, though it can vary slightly from app to app: 
For a list of apps on different mobile devices see the Zotero for Mobile support page.
