We say something is going to happen when it has already been planned.
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.
"We're going to buy a new car next year."
We also use going to when we can see something is about to happen.
"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.
"It looks as though Manchester United are going to win the European cup.
"I think my friend Louise is going to have a baby."
When we give information about the future or predict future events that are not certain we usually use shall/will.
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.
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.
The party will start at 10.00pm.