Getting over shyness, help?

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Ok, so my cousin is 12 (almost 13) and she's really self-conscious of herself (and I mean really). Like, she gets all tense whenever she sees someone she doesn't know. I asked her why she was so shy and everything, and she said maybe it was because she has a low voice (for a girl). Her mom yelled at her yesterday because my cousin's friend asked her over but she didn't know what to say because her other friend was already coming to her house, and my cousin's mom got mad because she had to tell the mom all that and all my cousin did was stare at the ground. Sometimes when she meets new people, she gets all silent. She says she's really afraid of what everyone will think, and her mom thinks it's because she doesn't smile, which might be part of it, but she's so nervous about what others thing. People get the wrong idea and think she's either dumb or impolite. Does anyone know how to get over shyness?