Talking about the future in Spanish is very easy and quite similar to the going-to future in English. As in English, Spanish uses the auxiliary verb ir (English: "to go"). Isn't that a beautiful coincidence?
I'm goingto call the police.
Voy a llamar a la policía.
Are you going to eat that?
¿Vas a comer eso?
She is not going to come.
(Ella) no va a venir.
the preposition a
the conjugated form of the verb ir
the verb that actually explains what is going to happen in the future (here: llamar, comer, venir), is in infinitive, i.e. not conjugated.
These are the forms of ir
Voy a comprar unas cosas al supermercado.
I'm going to buy some stuff at the supermarket.
¿Qué vas a hacer el fin de semana?
What are you going to do on the weekend?
This form is widely used in restaurants to talk about your order. So the waiter/waitress might ask: