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.