This is a practical oversimplification which will be right most of the time. For more detailed information see Mastery of the Tenses!
"going to do"
For general future plans, often when nothing fixed has been organised.
- I am going to the cinema next week.
- She is going to study Law.
For predictions based on observable data.
- It's going to rain! Look at those clouds!
- That room is going to look great when you finish!
For predictions based on things you know.
- This exam is going to be really difficult.
- They are going to be late if they leave now.
For questions when the other person probably knows the answer.
- What are you going to cook for dinner?
- When are you going to arrive?