How do i come out

Im in a very strange position i know he would accept me he's a big part of the LGBTQ community I just dont know how to word it he hasent lived with me since I was 4 im 15 and we talk sometimes but not as much as we probably should and its more because of my situation the two adults that I do live with one of them I don't think would really be mad or confused he would accept me more than likely but my mom is different she has made it clear that her beliefs are that transgenderism is a mental illness and government control and I can't tell her but I want my bio dad to k own im just not sure how to word it properly