Here are some possible answers, depending on the context and your intention:
If you are going:
* Sí, voy al cine la próxima semana. (Yes, I'm going to the cinema next week.)
* Sí, voy a ver [nombre de la película] la próxima semana. (Yes, I'm going to see [movie name] next week.)
If you are not going:
* No, no voy al cine la próxima semana. (No, I'm not going to the cinema next week.)
* No, no tengo planes de ir al cine la próxima semana. (No, I don't have plans to go to the cinema next week.)
If you are unsure:
* Tal vez, ¿qué película quieres ver? (Maybe, what movie do you want to see?)
* No lo sé todavía, ¿y tú? (I don't know yet, what about you?)
You can also use more informal answers like:
* ¡Claro que sí! (Of course!)
* Sí, por supuesto. (Yes, of course.)
* No, no creo. (No, I don't think so.)
* Quizás. (Maybe.)
The best answer will depend on your relationship with the person asking the question and the level of formality you want to use.