How can i thinnen long hair?

So im getting my hair cut soon and my hair is just so thick when i straighten my hair (GHDS) it wont go dead sraight at the bottems cos my hair poofs. So i want to get my hair thinnened so the end will go dead straight. But the problem is im not allowed to get it thinnened only layed to make it thin but i want to keep my haair long is there a certain layered style and stuff to do so please help?