How would I go about coming out as bi?

I'm a 15 year old who identifies as bisexual and male. I attend a catholic school, and I was raised in a VERY conservative household, and homosexuality is viewed as a sin by my family. I used to be catholic, until I underwent a deep depression and picked myself back up by dropping religion. Anyways, my family is starting to see that I'm not turning out how they raised me. My mom pesters me with questions and I'm pretty sure she has suspicions that I'm not straight. I've defended the lgbtq community many times at family events and started a few arguments about it, and I've only told my one cousin who isn't a complete dolt. I just wanna get this over with, but I know nothing will be the same between me and my family. My grandfather, who I love, will probably hate me for it. Anyone have any suggestions or help?