The causes of hair loss in a teenage girl is also similar to that of a mature lady.
However, the more dominant causes are as mentioned by Dr Colin, mainly Telogen Effluvium. Another common cause is Traction Alopecia, due to tying hair too tightly.
Other causes like Autoimmune Disease ( SLE, Thyroid conditions) and Nutritional Deficiencies are common and need to be ruled out as well via blood tests and a physical examination.
It’s best to seek the consultation of a doctor/dermatologist experienced in hair loss diagnosis and treatment.
If not, you can try buying Minoxidil 2% from the pharmacist and observe the hair loss condition after using it for 3-6 months, by which any telogium effluvium would have recovered usually.