A. Intentions

We use be going to to talk about something we have decided to do (an intention). David intends to climb up the ladder.
Here are some more examples.
I’m going to watch the next programme.
Emma is going to do an experiment this afternoon.
Rachel and Vicky are going to spend six weeks in the State:.
We can use I’m not going to for a refusal.
I’m sorry, but I’m not going to walk half a mile in the ram.
(= I don’t want to/I’m not willing to walk.)
The present continuous can have a very similar meaning to be going to. We can often use either form.
I’m going to visit my friend at the weekend.
I’m visiting my friend at the weekend.
We do not use will here.
We can use be going to with the verb go (We’re going to go
out this evening), but the present continuous is more usual.
We’re going out this evening.

B. Form

We use the present tense of be + going to + a verb.
They’re going to move house. Matthew is going to play squash.
Vicky isn’t going to have any lunch. We aren’t going to complain.
Is Daniel going to apply for the job? ~ / think he is.
When are you going to pay this bill? ~ I don’t know. I can’t at the moment.

C. Predictions

We also use be going to for a prediction based on the present situation, when we can see that something is going to happen. The ladder is moving, so David is going to fall.
Here are some more examples.
My sister is going to have a baby in March.
It’s nearly nine now. We’re going to be late.
Do you think it’s going to rain?

 

Source : Oxford Practice Grammar