Why do you feel it is necessary to make a big announcement? Just let your actions speak for themselves, and if asked, then quietly discourse with the questioner.
I believe in very little that my family do - god exists, jesus lived died and was reborn then taken to heaven, newspapers don't lie, the government cares about people, a 'good' funeral send off, being nice to people,even if you don't like them, drugs are always bad (unless a doctor says they are good), There is so much to argue about so I rarely speak with them.
As with anything it depends on how open they are. Some are fortunate to just come right how, others have to hide it. Like the proverbial Bandaid you may just to do it snd quickly.
Test the water and see and be prepared for questions or even damnation. Hopefully your family will understand.
We're here if you need support either way.
My dad was never a believer. He admitted that he took us to church all those years(plus was a deacon, treasurer, in the choir etc) because it seemed like ‘the right thing to do’.
I never believed either. I refused church around age 16. I just told them straight. A few of the family think I can be ‘saved’ but we have a deal: they don’t preach at me, and I won’t present facts to them.
Interesting encounter with a very religious cousin. She said she was so worried and asked: but where will you spend eternity?
My answer: same place you will.
"Mom, I decided I am an atheist," I said at 13. "I don't want to go to church anymore."
"That's fine, honey," she replied.
My mother became an atheist in nursing school.
"I realized a woman cannot be turned into salt," she said dryly.