I'm bisexual myself and haven't told one of my parents. It's a decision that's up to you. If you're really comfortable telling them and you're completely sure of your sexuality, something that often isn't the case (especially during puberty, to which I can attest), then I suppose it may be up to circumstance and how you think you should proceed. I don't know your full situation, but if you think you're more comfortable staying closeted, that's always an option as well! There's no rush to out yourself, and, if you do want to, maybe talk to a trusted friend or other accepting adults before your parents to try and figure out what your next step forward would be.
I know you might feel a bit uncomfortable talking about it, but there's no reason to be, this is just one part of you, and it's totally alright to be who you are. You can simply open up and try to explain them how you feel, being as honest and open as possible, encouraging them to ask you anything they don't understand or are unsure about. Make sure they know you're there to help them understand you. Remember, the people who truly love us always be by our side, no matter what. If you give them the chance to understand you, they can become your greatest supporters!