apple ID

I forgot i already had an apple id, got a new phone which asked me to set up my apple id - so i put in the wrong email address (which doesnt actually exist), so i dont think they could have verified my apple id.

So now i have to return the phone and disable 'find my iphone' - i cant find the sim for the phone as i removed it a few days ago

How do i disable 'find my iphone' - i think my original apple id is still working

thanks!