Tbh I think they know about you already!
And you should be really happy to have supportive parents cuz not everyone in the world has that! (I mean I do cuz my parents are awesomesauce but not everyone has awesomesauce parents)
Pick a time when they are both free to say, "Hey mom, dad, can we sit down and have a talk please?" Then once you have them you say, "Guys.. The reason I've been talking about pride and LGBTQIA+ stuff is because I'm bisexual.."
Or something like that!
I, personally, am cisgender and most likely straight, so the title doesn't apply to me, but I hope I could help!
I'm bisexual too
And Kevin over there (The weirdo who edits my things) is also bi so he supports you too!
:DD
Good luck, my fren!
-Nobi
PS., KEVIN IS SORTA KINDA NOT A REAL PERSON JUST SOMEONE I MADE UP BUT WE TREAT HIM LIKE A REAL PERSON BUT IF HE WAS NOT JUST A FIGMENT OF MY IMAGINATION HE WOULD MOST LIKELY BY BI PROBABLY I DON'T KNOW JUST KNOW WE SUPPORT YOU K BYE—