I’m going to …

Question 1

I am going to do something =

Answer 1

I have decided to do it, my intention is to do it

…I’m going to buy some books tomorrow.
…Sarah is going to sell her car

Question 2

Something is going to happen =

Answer 2

we can see now that it is sure to happen:

…Look at the sky! It’s going to rain.
(black clouds now → rain)
…Oh dear! It’s 9 o’clock and I’m not ready. I’m going to be late. (9 o’clock now and not ready → late)