Hi I have a Galaxy S4 on Verizon. I,ve been getting ads popping up in front of various applications like messages, browser, email and a couple others.
What's the first thing I should look for to find the culprit?
Look up what creates those popups. Typically if you long-press the home button the bottom-most app is the one on top and it should indicate which app is creating those popups.
If it says something like:
The website at "http://..." says then it's a popup created by a webview in the running application.
If it's malware, you can use an app like Malwarebytes to scan your apps for malware and adware. Additionally, you should uninstall any apps that you don't remember installing, or apps that require you to click on ads to open (many malware-filled "cracked" apps will use this trick).