When we want to talk about future facts or things we believe to be true about the future, we use 'will'.
The President will serve for four years.
The boss won't be very happy.
I'm sure you'll like her.
I'm certain he'll do a good job.
If we are not so certain about the future, we use 'will' with expressions such as 'probably', 'possibly', 'I think', 'I hope'.
I hope you'll visit me in my home one day.
She'll probably be a great success.
I'll possibly 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'.
Not a cloud in the sky. It's going to be another warm 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.
Be careful! You're going to spill your coffee.
At the moment of making a decision, use 'will'. Once you have made the decision, talk about it using 'going to'.
I'll call Jenny to let her know. Sarah, I need Jenny's number. I'm going to call her about the meeting.
I'll come and have a drink with you but I must let Harry know. Harry, I'm going to have a drink with Simon.