How do I disable my stolen iphone 8plus ??

Hello ~ You can follow the steps here…

If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

We are fellow forum members like yourself and you will have to take the proper steps in the article above.

~Katana-San~

Apple can’t disable your phone. If you had Find My turned on log in to https://icloud.com/find, click on the phone in the list of devices, and click Lost Mode.

Beware of a common phishing scam: You may get a text or email from “Apple” or “Apple Security” (or sometimes “Police”) saying your phone has been found and to click on a link to see its location. This is NEVER from Apple or from law enforcement; it is from the thief or fence to steal your Apple ID and identity. Delete any such message without responding to it. Apple never gets involved in lost or stolen devices→Avoid phishing emails, fake ‘virus‘ alerts, phony support calls, and other scams