How to get my hair to grow thicker?

SO, i want my hair to get thicker. Im sick of having semi thin hair. Its not like thin thin but its pretty thin. When my mother was little she lost all of her hair because of allapecia however you spell that. That is not happening to me but im sure i got my moms genes for thin hair. Its not fair because my sister got my dads hair which is thick as crap. It makes me upset feeling ugly sometimes because my hair isnt naturally thick. So i need tips and ideas and things i can do to get my hair thicker. Im 13 going on 14 soon. Name anything and yes im allowed to get stuff from GNC also. Ive heard drinking alot of milk and taking fishoil capsules help. Please help:) Thanks<3