How to Come out

So I’m 16 and I’m gay I haven’t told anyone this yet but i really wanna tell my mom but I just don’t know how to especially because I know she’ll tell my family who’s transphobic and homophobic but I feel like her and my sister already knows but we’re going out to spend time together and I really wanna let her know because I’ve keep this from her since I was 10 but if you guys have any ideas on how I could tell her that would be really helpful.