I am going to do something = I have already decided to do it, I intend to do it:
A : Are you going to watch the football game on TV tonight?
B : No, I’m going to go to bed early. I’m tired from my trip.
A : I heard Lisa won some money. What is she going to do with it?
B : She’s going to buy a new car.
I’m going to make a quick phone call. Can you wait for me?
This cheese smells awful. I’m not going to eat it.
I am doing and I am going to do
We use I am doing (present continuous) when we say what we have arranged to do – for example, arranged to meet somebody, arranged to go somewhere:
What time are you meeting Amanda tonight?
I’m leaving tomorrow. I already have my plane ticket.
I am going to do something = I’ve decided to do it (but perhaps not arranged to do it):
“The windows are dirty.” “Yes, I know. I’m going to wash them latter.”
(= I’ve decided to wash them, but I haven’t arranged to wash them)
I’ve decided not to say here any longer. Tomorrow I’m going to look for another place to live.
Often the difference is very small and either form is possible.
You can also say that “something is going to happen” in the future. For example:
The man can’t see the wall in front of him.
He is going to walk into the wall.
When we say that “something is going to happen,” the situation now makes us believe this.
The man is walking toward the wall now, so we can see that he is going to walk into it.
(situation now) — going to –> (future happening)
Some more examples:
Look at those dark clouds! It’s going to rain. (the clouds are there now)
I feel awful. I think I’m going to be sick. (I feel awful now)
The economic situation is bad now, and things are going to get worse.
I was going to (do something) = I intended to do it, but didn’t do it:
We were going to fly to New York, but then we decided to drive instead.
Peter was going to take the exam, but he changed his mind.
I was just going to cross the street when somebody shouted, “Stop!”
You can say that “something was going to happen” (but didn’t happen):
I thought it was going to rain, but it didn’t