fake virus notification and update flash player popups
I have read everything I can find on this and nothing seems to work for removing what I assume is malware. I keep getting the popup that tells me to install the update for flash player (not legit). I can't find anything "suspicious" to remove when I look in my applications. In addition, I keep getting the fake message that my computer is infected with a virus. Again, I can't seem to find the source. I've looked in System Processes and there are so many, I'm afraid to quit any and none are obvious. I'm open to downloading something to find it automatically but am afraid of choosing an application that causes even more issues. I prefer to do it manually but I can't find anything obvious to remove. I did recently upgrade to Mojave hoping it would remove the problem but it didn't. Eternally grateful to anyone who can help with this!
Wasn't sure which community to ask this in so let me know if you suggest another one.
MacBook Pro (13-inch Mid 2012), macOS Sierra (10.12.6)