Who am I telling?
I dont know why you use this question , but you should say " Whom am I telling?
Just like "Whom are you calling " ; not "Who are you calling "
" Whom am I telling? = I am telling whom ?"
The verb "telling " points to an OBJECT here ,which is the person you are "telling to:...
> The reciever of the verb is the object . So, if there is an object for the verb, you use "whom".
If you are keen , check the Internet for "who vs whom"