how do i know if a virus alert is real and what should i do?

How do you tell it's fake?
It showed up - that's all you need to know how it's fake.

If problems return and you are using Safari see this article

Clear the history and cookies from Safari on your iPhone, iPad, or iPod touch - Apple Support

In addition to what Jimmy told you, I would recommend to install an Adblocker like this one: Adblock Plus (ABP): Remove ads, Browse faster without tracking on the App Store since I use this, I never receive this crap anymore.