Help With My Thick, Unmanageable hair?

So I have hair that is wavy underneath and pretty straight at the top. I dunno why, but yeah. It's thick and I can't curl or scrunch it. It's easier to just straighten it, but it doesn't like to stay even if I use straightening serum, hairspray, and whatnot. & I can't just leave it because it's just bad. Help please.