Why does my hair tangle so easily?

When I get out of the shower, I flip my head forward and light tousle with a towel and leave it up for a few minutes. When I take it out, my hair is EXTREMELY tangled and knotted, and I don't know why. When I try to brush it out, a ton of strands of my hair come out and I'm starting to be scared. I don't want my hair to continually fall out. I'm only 13. How do I prevent my hair from tangling or untangle it when it does knot up? )':