Sharing some notes I have on this topic:
The Basic Rule of WILL and GOING TO:
We use ‘going to’ to talk about something we have planned. ‘going to’ = ‘planning to’
We use ‘will’ to talk about a spontaneous or unplanned future event.
He is going to buy a new car. (He is planning to buy a new car)
I think I will take the bus today. (I have just decided)
When we want to talk about future facts or things we believe to be true about the future, we use 'will'.
The boss won't be very happy.
I'm sure you'll like her.
I'm certain he'll finish it today.
If we are not so certain about the future, we use 'will' with expressions such as 'probably', 'possibly', 'I think', 'I hope'.
She'll probably be a big star someday.
I'll try to come but I may not get back in time.
I think we'll get on well.
If you are making a future prediction based on evidence in the present situation, use 'going to'.
The sky is clear. It's going to be sunny day.
The traffic is terrible. We're going to miss our flight.
Be careful! You're going to spill your coffee.
However, both "will" and "be going to" can express the idea of a general prediction about the future.
It looks like she's going to win.
It looks like she'll win.
Notes are from: HERE
Practice quiz: HERE