The best way to understand is through examples. Take this conversation, for example. Two friends are in a restaurant, looking at a menu. B knows what he wants to order, but A hasn't decided yet. 1. A: Have you decided yet? 2. B: Yes. 4. A: So, what are you going to order? 5. B: I'm going to have steak and fries. What about you? 6. A: I'm not sure. Maybe I'll just have a salad. 7. B: You'll be hungry later if you only have salad. 8. I'll share my fries with you if you want. 9. A: Thanks, but I'm trying to lose weight. 10: B: Look, there's sushi on the menu. Why don't you have that? 11. A: Oh, yes.Good idea. I'll have the sushi. lines 4 and 5 : They use 'going to' because they're talking about something that has already been decided. It's a plan/ intention line 6: A is unsure, so uses 'maybe ..will' line 7: This is a first conditional, so B uses 'will' . (Though 'going to' would also be correct here). line 8: This is an offer, so B uses 'will' line 10: This is a new decision, so A uses 'will' I agree with everything that the other members have already said. Dina has given you some textbook rules, but as James says, these rules are not hard and fast, and in many cases you could use either form (or indeed other forms entirely). And as Frances says, 'going to' is used more in conversation, whereas 'will' is more of a feature of formal and written contexts. I hope this helps.

"Going to" talks about plans for the future. (for example, what are you doing tonight? I'm going to see a movie with my friends.) "Will" is a prediction. (I think I will arrive at 4:00PM.) "Will" is also used in the first conditional (If it doesn't rain, I will arrive at 4:00PM.) "Will" can also be a promise (I promise I will arrive on time!) I would suggest using "will" because time of arrival is more of a prediction, and "going to" is used more for making general plans.

I agree and would add that I think "I am going to arrive" is used much more in informal speech than "I will arrive".

I think they are very similar. The examples given by Dina below are not hard rules and in each of her examples, you could change "I will arrive" to "I am going to arrive" and it would be grammatically correct and you would be understood.