Cake can't hurt.
Do you live with them? If not, just do it gently. If so, I recommend waiting until you're old enough to leave if they take it particularly badly.
Well, it isn't like you're going to have all kinds of female partners at your age. There's really no need to make it a thing until you're a bit older.
make a video
Walk into the room making out with the most dyke looking girls you can find.
"hey guys im a rugmuncher and going to hell, seeya"
why do you have to tell them?
Tell them that your just another stupid kid who has been jaded by mainstream telling you it's okay to not be okay.
Why tell them at all?
Fair enough. Next time you talk with them, offhandedly mention a friend of yours recently came out of the closet. Tell them you aren't sure how to react to something like that. How would they have reacted? Depending on their answer can tell you how they may react to you telling them you're lesbian. But before that, I'd like to point out that if you are 13-15, it could just be a phase. COULD be. I'm not saying it is. Just every woman nowadays goes through such a phase to see if they prefer women over men. And at such a young age you could be "easily impressionable". I recommend first experimenting if you haven't already to see how you feel about it. But, this is your life and you'd know more about it than some some faceless, voiceless stranger on the internet(me). I wish you the best of luck.