Cómo se llama? is more common than Cual es tu nombre? Both are correct, but Cual es tu nombre? is not as usual as Cómo se llama? Appart from that I would reccomend you to use: ¿Cómo te llamas? It's less polite in the way of more commonly used. ¿Cómo se llama? You ask it to someone older or someone who deserves a great respect. ¿Cómo te llamas? Is the way we ask the name to someone in the usual way.