@ Kate_hehe I'm bi and I want to remind you that you never have the responsibility to tell ANYBODY your sexuality, 100% understand if you do want to though!!<33
I myself did it casually and made it fit into the subject we were talking about at the minute. I don't remember exactly what context it was but maybe something like "blah blah, and if you're bi, like me," and then continue speaking. Depends on how your relationship w your parents is ofc, in another case you can start off by saying "I've started to realise I can be attracted to both boys and girls (and possibly people in-between if that's the way it is for you)" and basically just explain the term to them and what it means for you. Hope it helps!!<333