How do I sign into my iCloud on another iPhone, when there’s someone else’s account signed in?

For what purpose are you trying to sign into your iCloud account? Is this a used phone that you're trying to set up for yourself? If so, the person who's currently signed in will need to sign out using their password.

If you're trying to use Find My... to find a device connected to your Apple ID, see here:

If your iPhone or iPad is lost or stolen - Apple Support

If you mean something else, please explain.