I am 17 years old and a dark spot appeared on my breast and i wanna know if it may be something dangerous or a breast cancer?

Reassured: I think it is a very low likelihood to be breast cancer. Breast cancer would present itself to you as a lump deeper in your breast, not as a dark spot. What you describe sounds more like a mole. If the dark spot is growing or otherwise changing, see a doctor about finding out if it needs to be romoved.

Unlikely: Breast cancer is very unusual in a 17 year old young lady it is more likely that you have a benign skin nevus or freckle if you are worried please see you doctor.

See your derm: Any new skin lesions should be brought to the attention of your friendly dermatologist.