Will or Going to

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.