USE
1.-To talk about future facts / things we believe to be true about the future:
The teacher won't be very happy.
I'm sure you'll pass all your exams
2.-If we are not so certain about the future, we use 'will' with expressions such as 'probably', 'possibly', 'I think', 'I hope'.
I don’t think I’ll go out tonight.
I promise I won’t tell Montse what you’ve just said
She'll probably pass her driving test this year.
3.-At the moment of making a decision, use 'will'.
I’ll have a mushroom pizza, please.
I know! I’ll send him an email.
USE
1.-Once you have made the decision:
I'll call Joan to let him know. Paula, I need Joan's number.
I'm going to call him about the meeting.
She isn’t going to study at University next year.
2.-To make a future prediction based on evidence in the present situation:
Not a cloud in the sky. It's going to be a nice day.
Look at the queue. We're not going to get in for hours.
The traffic is terrible. We're going to miss our flight.
That boy can’t skateboard. He’s going to fall over.