You should tell them, but are you sure you are bi? And how old are you? If your 13 or under, I don’t recommend coming out quite yet. Things change as you get older. If you think you will be fine and will stick to bi, then I can give you a couple tips. I have not come out myself but my best friend did.
You have to go up to them and just admit it. You can make it private but I’d just randomly tell your parent “So… I need to tell you something…”
GO TO THE LEAST HOMOPHOBIC ONE FIRST. MY BESTIE WENT TO HER DAD THINKING HE WOULD BE FINE AND HE MIGHT HAVE FREAKED. AND THEN HE BROKE THE NEWS TO HER MOM HIMSELF. SHES FINE NOW BUT THAT IS DEFINITELY NOT THE WAY TO GO. OR GO TO BOTH. IDK THOSE TWO WORK.
When you tell them, you tell them sincerely and look them in the eye. It’ll be okay to look away after. They know to support you even if thats weird as heck to them and should at least respond with something. Whether that’s “What does that mean?” or “Aww... honey… it’s alright…” thats something sort of positive.
That’s all I got but I hope it helps.