If you are under the age of 18 and have no means of supporting yourself and no way of leaving the state of Utah then I would recommend keeping your religious preference to yourself at this time. There is no need to go through a coup just to be stuck in Mormon country and forced to deal with the religious aspect of your life and scrutiny for attempting to be different.
If however; you have the means to get out or you don't see it as being a HUGE deal with your parents then... maybe you bring it up casually. Just mention that you're not completely in agreement with the Book of Mormon and go from there.
For me - it was a lot different when I told my father I didn't believe in the Catholic Faith or in a God at all. I was thrown out as a result of an abortion which went heavily against his faith. But, for me - it wasn't something I could live with so I chose to live without a family.
I know not the best story of "hey good luck" but, I'd be willing to talk to you about ways to communicate bad news. Make an account and personal message me.