Don’t worry at all, if you’re talking about having an older sister, you sound as if you’re a teenager. If you don’t know what masturbation is, it’s nothing you have to learn.
It’s purely and simply about giving yourself pleasure, making yourself feel nice by playing around with yourself. You could do it with anything you find pleasing. T
hink about how lovely it is to have your hair tickled or your head massaged, or your skin stroked, and try to recreate those sensations by playing around with your clitoris and vagina, your breasts and nipples - and experiment with fingers, toys, ice, warmth, different speeds, rhythms, try it with or without music, in the dark or with a mirror in the light, in water or just somewhere private.
Some girls start off masturbating together, in a totally innocent, explorative way, when they’re pre-teen, and it just becomes something that’s as natural as scratching your head.
It’s not something to worry about - if you’re discovering it slightly later, that’s fine. When you start to get to an age where you feel more sexual and want more satisfaction, you might explore how to make yourself orgasm, and that might involve you ejaculating some warm liquid, as you would if you were having sex - so it becomes having sex with yourself.
But it doesn’t have to have a goal or a result. Just make sure that before touching yourself in intimate parts, your hands are clean so that you avoid giving yourself any infections.
Your sister may or may not be comfortable talking about it - but you could always do something subtle like leave a diary open somewhere she’ll read it, and say you wish you could talk to her about it.
Give her the option to come to you. I don’t know how close you are and how much you trust her not to blab about your worries.