Sounds like you either have a virus, malware, or bad app on the phone that you need to remove. Or it could even be that you installed an app that allows for ads (so many do) and that app is the cause. To be sure that is what it is, try starting up the phone in "Safe Mode". If you get no popups in there, then it is definitely one of the above. You would need to then check the device and remove whatever is causing the problem.
To get into "Safe Mode" please do the following.
1. First turn off the phone.
2. Next press and hold "Power" until you see the manufacturers logo on the screen, then release the power button.
3. "Quickly" press "Volume Down" and hold it for few seconds until you see the message "Safe mode: ON" or something similar.
4. If you wanted disable it, simply repeat the steps above, and you should see "Safe Mode: OFF" rather than on.