When we want to talk about future facts or things we believe to be true about the future, we use 'will'.
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The President will serve for four years.
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The boss won't be very happy.
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I'm sure you'll like her.
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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'.
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I hope you'll visit me in my home one day.
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She'll probably be a great success.
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I'll possibly come but I may not get back in time.
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I think we'll get on well.
If you are making a future prediction based on evidence in the present situation, use 'going to'.
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Not a cloud in the sky. It's going to be another warm day.
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Look at the queue. We're not going to get in for hours.
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The traffic is terrible. We're going to miss our flight.
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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'.
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I'll call Jenny to let her know. Sarah, I need Jenny's number. I'm going to call her about the meeting.
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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.