The most common method of aquiring adware is through downloading an app.
To see whether you are using an app that renders advertisments you can boot your device into safe mode.This will prevent any third party software from running.Since i do not know which device you are using you can try this to boot into safe mode. How To Boot Your Android Phone or Tablet Into Safe Mode
If the problem persists once in safe mode it is not an app.
If it does not persist you will have uninstall an app to remove the adware.