virus warning that I have a Trojan Virus on my iPhone 7

I just received a message on my iPhone 7 that I had a Trojan Virus on my iPhone prompting me to hit ok. I hit ok and then panicked and shut down phone immediately. Is this a scam?

I turned phone back on and went into settings and cleared history and website data.

Is this a legit virus and if so, what do I do to rid my iPhone of this?

Please help.

Susan