going to vs will

Going to

We say something is going to happen when it has already been planned.

For example:-

Q) Are you going to fly to Prague?
A) No, we're going to drive.

We use it to show something has already been decided.

For example:-

"We're going to buy a new car next year."

We also use going to when we can see something is about to happen.

For example:-

"Look at that cloud. I think it's going to rain."

"Watch out! He's going to crash into that tree!"

You can also use going to to predict the future based upon the evidence now.

For example:-

"It looks as though Manchester United are going to win the European cup.

"I think my friend Louise is going to have a baby."

Will/shall

When we give information about the future or predict future events that are not certain we usually use shall/will.

For example:-

Q) Who do you think will win the election?" A) "I'm not sure but I think the current party will win."

We can also use shall/will to make promises for the future.

When leaving work I would say - "Goodnight, I'll (I will) see you tomorrow."

Shall/Will is often used when we just decide to do something.

For example:-

The phone is ringing - If I decide to answer the phone I would say - "I'll (I will) get it."

It can also be used in formal situations to express planned events and is preferred in formal written English.

For example:-

The party will start at 10.00pm.