I am 21years, i am underweight, 179cm of height, i am too skinny my hair growth is not normal i wonder if i need to take exogenous testosterone.

Go see your doctor: There are many reasons for being underweight (you didn't state how much/little you weigh). Typically we calculate body mass index (http://www.cdc.gov/healthyweight/assessing/bmi/adult_bmi/english_bmi_calculator/bmi_calculator.html) such that less than 18.5 is considered underweight (or less than 59.2kg). Regardless, you need to go see family doc for complete evaluation before starting treatment