Future forms

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

My sister's going to adopt a child.

Are you going to buy a new car or a second-hand one?

I'm not going to go home for the holiday. It's to far.

Predictions

We also use be going to to make a prediction about the future, especially when we have some evidence.

Barcelona are going to win. They're playing really well.

Look at those black clouds. I think it's going to rain.

There is very little difference between the present continuous and be going to for future plans / arrangements and often you can use either.

Be going to shows that you have made a decision: 

We're going to married next year.

The present continuous emphasizes that you made the arrangements: 

We're getting married on October 12th. 

In this example, we've booked the church.

We often use the present continuous with verbs relating to travel arrangements, e. g. go, come, arrive, leave, etc.

He's arriving tomorrow and leaving on Thursday.

Will / shall: instant decisions, promises, offers, predictions, future facts, suggestions

We use will / won't for instant decisions, promises, and offers.

I'll have the steak.

I won't tell anybody where you are.

I'll carry that bag for you.

We can also use will / won't for predictions and to talk about future facts.

You'll love New York

I think Barcelona will win.

Next year Christmas Day will be on a Thursday.

The election will be on 1st March.

We use shall (NOT will) with I and we for offers and suggestions when they are questions.

Shall I help you with your homework?

Shall we eat out tonight?

The exam will be on the last Friday of term.

Shall we invite your parents for Sunday lunch?

It's Alice's birthday tomorrow. Oh, really? I'll make a cake.

I'm not havind dinner with my family tonight.

I'm not going to have dinner with my family tonight.

Are you going to go to Jo's party next weekend.

You can trust me. I won't tell anyone what you told me.

What are you doing on Sunday?

We're visint my grandparents.

I think the birth rate will go down in my country in the next few year.

My grandfather is going to retire at the end of the month.

My granfather is retiring at the end of the month.

Shall I help you with the washing-up?

What's your mother going to do about her car?

She's going to buy a second-hand one.

I'm goign to miss you. Please stay in touch.

I promise I will phone every day.

What are Alan's plans for the future.