Para que se utiliza "going to":

1. Para hablar sobre intenciones en el futuro

- "I'm going to travel." (Voy a viajar.)

          -"He's going to lose weight." (Va a adelgazar.)
        
          - "I am going to have lunch with the boss tomorrow." (Voy a comer con el jefe mañana.)

2. Para hablar sobre predicciones basadas en mucha evidencia. 

- "The weather report says it's going to rain this evening so bring your umbrella." (El informe sobre el tiempo dice que va a llover esta tarde así que trae tu paraguas.)                   

-"The Economist" says the price of gasoline is going to go up so buy a more fuel-efficient car." ("The Economist" dice que el precio de la gasolina va a subir así que compra un coche más eficiente.)      

La estructura básica del "going to":

La estructura básica del afirmativo del "going to": (somebody=alguien; somewhere-algún sitio; something=algo)

	suj	aux	going to	verbo
	I	am	going to	eat	something.	Voy a comer algo.
	You	are	going to	talk	to somebody.	Vas a hablar con alguien.
	He	is	going to	listen	to something.	Va a escuchar algo. (él)
	She	is	going to	read	something.	Va a leer algo. (ella)
	It	is	going to	go	somewhere.	Va a ir a algún sitio.
	We	are	going to	look	at something.	Vamos a mirar algo.
	You	are	going to	wear	something.	Vais a poneros algo.
	They	are	going to	sit	somewhere.	Van a sentarse en algún sitio.

La estructura básica del negativo del "going to":

	suj	aux	not	going to	verbo
	I	am	not	going to	eat	something.	No voy a comer algo.
	You	are	not	going to	talk	to somebody.	No vas a hablar con alguien.
	He	is	not	going to	listen	to something.	No va a escuchar algo. (él)
	She	is	not	going to	read	something.	No va a leer algo. (ella)
	It	is	not	going to	go	somewhere.	No va a ir a algún sitio.
	We	are	not	going to	look	at something.	No vamos a mirar algo.
	You	are	not	going to	wear	something.	No vais a poneros algo.
	They	are	not	going to	sit	somewhere.	No van a sentarse en algún sitio.

Nota: Contracciones: Se suele contraer "It is" con "it's", "it is not" con "it's not" o "it isn't", "you are" con "you're", "we are not" con "we're not" o "we aren't", "that is" con "that's", "that is not" con "that's not" o "that isn't", etc. (Nota: "I am not" solamente se contrae con "I'm not").
     
La estructura básica del interrogativo del "going to": (anywhere = alguna o cualquier parte, anybody = cualquiera, alguien, anything = cualquier cosa, algo)

	aux	suj	going to	verbo
	Am	I	going to	eat	something?	¿Voy a comer algo?
	Are	you	going to	talk	to somebody?	¿Vas a hablar con alguien?
	Is	he	going to	listen	to something?	¿Va a escuchar algo? (él)
	Is	she	going to	read	something?	¿Va a leer algo? (ella)
	Is	it	going to	go	somewhere?	¿Va a ir a algún sitio?
	Are	we	going to	look	at something?	¿Vamos a mirar algo?
	Are	you	going to	wear	something?	¿Vais a poneros algo?
	Are	they	going to	sit	somewhere?	¿Van a sentarse en algún sitio?

La estructura básica de las respuestas cortas del "going to" (e iguales a las del verbo "to be"):

	Af.					Neg.
	Yes,	I am.				No,	I'm not.
	Yes,	you are.				No,	you aren't. (you're not)
	Yes,	he is.				No,	he isn't. (he's not)
	Yes,	she is.				No,	she isn't. (she's not)
	Yes,	it is.				No,	it isn't. (it's not)
	Yes,	we are.				No,	we aren't. (we're not)
	Yes,	you are.				No,	you aren't. (you're not)
	Yes,	they are.				No,	they aren't. (they're not)