My 12 year old daughter smoking ? what do i do?

ok so my daughter had a really naisty cough for about 3 weeks she would cough till she was sick about 3 times a day so i took her to the doctors and they said she had a chest infection caused by smoking and she told me she didnt smoke but i took her for nicotine test and they found nicotine in her system then she came clean with me and told me she had smoke since she was 8! i know this sounds really bad and you must think im a really bad parent but i had no idea she told me one of her friends mums buys then for her and her friends but she will not tell me which mum she tells me she is very addicted and cannot stop but i can see how seriously it is effecting her health what do you think i should do i need advice