Hello Garyn, welcome to the forums! If this is happening only when using a browser you could try clearing cache and data for it in settings - apps and see if it helps. If the ads pop up no matter what you're doing you need to find out which apps is the culprit. There are many possibilities, file explorers, keyboards, free games, etc. Apps like ES File Explorer, Z camera, DU or CM boosters, GO apps, have been reported for pushing unwanted ads.
You can also try a permission manager from the Play Store to narrow the search to those with the 'draw over apps' permission.