Area on scalp the size of a pencil eraser with no hair ! I'm only 17 ! What is it ?!?

I was parting my hair yesterday and noticed that my hair looked thin in this one spot of my head. I parted it right along it to see what it was and a circle area about the size of a pencil eraser is missing hair! I'm only 17 y/o (just turned 17 at the start of may) and I'm really worried! I have no clue what it is at all! Any clues ?!?!