What happens if my phone says I have the Trojan virus, how can I get rid of it?

A non-jailbroken phone cannot get a virus. Those are scam messages that you should ignore.

For a start, there’s no such thing as a trojan virus. But see this Apple support link→Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

i’m pretty sure that you can’t get a trojan virus (also known as a trojan horse) on an IOS device unless you’ve jailbroken it before so whatever is telling you that may be a virus itself or just wrong

No, there is no such thing as a "trojan virus" aka a trojan horse. A Trojan horse and a virus are different things.