There are many causes of fatigue - everything from depression, anxiety, stress, medications, viral/bacterial illnesses such as mono (but unlikely at your age), thyroid dysfunction, carcinoma, infections, obstructive sleep apnea, anemia, electrolyte imbalance, obesity, heart failure, lung disease. Sometime people with fatigue can get memory impairment, sore throat, joint pains, headaches. You will unfortunately need to see a doctor to rule out any underlying medical condition such as thyroid disease, adrenal disease, diabetes, infections, sleep apnea, tumors/malignancy, autoimmune disease, and inflammatory disease which are all common at your age. Standard blood work should be done. There is no specific treatment or medication for fatigue; therefore, you need to find out if there is an underlying disease.