Welcome to Android Central! Are you seeing them only while using the browser, or are they appearing on your homescreen? Understand that these aren't ads created by Google--they're from the companies trying to sell you whatever apps show up. If it's only while browsing, they're probably popups associated with the particular website you're visiting--avoid that website. Also, make sure your browser's popup blocker is turned on (in the browser settings).
If they're showing up on the homescreen, then some app you installed previously contained some adware. You'd have to uninstall apps one by one until this behavior goes away.