How is tongue curling an inherited trait

It is like any other genetically inherited trait: there is a dominant allele and a recessive allele for this trait just as there is for other physical traits. The ability to roll the tongue is dominant, so you can roll your tongue as long as you have at least one dominant allele for it. However, if you cannot roll your tongue, you cannot learn. You either can or you cannot, and you inherit one allele for this from each parent. It is not a learned ability and you cannot teach yourself to do this.
It's like people who can wiggle their ears. Most of us cannot and it is an inherited, not a learned, trait. If you get the right alleles, you can wiggle your ears. If you don't, you never will.
Tongue curling is no different from any other genetic trait. You inherit the allele combination from your parents, and if you are heterozygous recessive (in this case), you will never be able to curl, or roll, your tongue no matter how hard you try. If you are homozygous dominant or heterozygous for this trait, you can curl your tongue and will be able to for the rest of your life. It's just another roll of the genetic dice.