Subject + am/is/are+ going to + Verb
This structure is very common in speaking. It is really a present progressive tense . We use it to talk about future actions and events that have some present reality. If we say that something in the future is going to happen, it is usually already planned or decided, or it is starting to happen, or we can see it coming now.

Going to

1. We use ‘be going to’ + infinitive when we say what we have already decided to do or what we intended to do in the future.

I am going to do something = I have already decided to do it, I intend to do it:

• • We’re going to get a new car soon.
  • Jack says he’s going to phone this evening.
  • When are you going to get your hair cut?

2. Another use of the going-to structure is to predict the future on the basis of present evidence – to say that a future action or event is on the way, or starting to happen.

• • She’s going to have a baby.
  • Look at the sky. It’s going to rain.
  • Look out! We’re going to crash!

3. Going to .. . can be used to insist that people do things or do not do things.

• • You’re going to finish that soup if you sit there all afternoon!
  • She’s going to take that medicine whether she likes it or not!
  • You’re not going to play football in my garden.

4. In informal speech, going to is often pronounced /дэпэ/. This is sometimes shown in writing as gonna, especially in American English.

• • Nobody’s gonna talk to me like that.

5. To say that something was still in the future at a certain past time, we can use a past form of one of the future structures.

• • We were  going to  take the train but then we decided to go by car.
  • A: did Alice take the exam?
    B: No, he was going to take it but then he changed his mind.

6. In predictions, we use going to when we have outside evidence for what we say – for example black clouds in the sky, a person who is obviously about to fall.

• • See those clouds? It’s going to rain.
  • Look – that kid’s going to fall off his bike.

We prefer will for predictions when there is not such obvious outside evidence – when we are talking more about what is inside our heads: what we know, or believe, or have calculated. (When we use will, we are not showing the listener something; we are asking him or her to believe something.) Compare:

• • Look out – we’re going to crash! (There is outside evidence.)
    Don’t lend him your car. He’s a terrible driver – he’ll crash it. (the speaker’s knowledge)
  • Alice is going to have a baby, (outside evidence – she is pregnant now)
    The baby will certainly have blue eyes, because both parents have. (speaker’s knowledge about genetics)

Will or Going to?

Will: We  use will when we decide to do something at the time of speaking. The speaker has not decided before.

Alice’s bicycle has a flat tire. She tells her father.

Alice: My bicycle has a flat tire. Can you fix it for me.
Father: Okay, but I can’t do it now. I’ll fix it tomorrow.

Here Alice told her father, he didn’t know about the flat tire.

Going to: We use going to when we have already decided to do something.

Later, Alice’s mother speaks to her husband.

Mother: Can you fix Alice’s bicycle? It has a flat tire.
Father: Yes, I know, She told me. I’m going to fix it tomorrow.

Here Alice’s father had already decided to fix the bicycle before his wife spoke to him.