Study materials
Be going to has two uses that look unrelated at first: plans and predictions. In fact they share one logic, and that is what makes the structure easy.
In both cases the future is already visible in the present. Either because the decision has been made, or because the signs are right in front of you.
I'm going to study tonight.
Look at those clouds. It's going to rain.
The decision is already made
The first use is a plan or an intention that existed before the conversation. You are not deciding now; you are reporting a decision.
We're going to move in September.
She's going to look for a new job.
They're going to sell the car.
Signs you can see right now
The second use is a prediction, but not just any prediction. It rests on what is visible this minute, not on a guess or a wish.
He's going to drop it!
The battery is at two percent. My phone's going to die.
Look at the queue. We're going to be late.
Notice that every sentence has the reason sitting next to it. The clouds, the battery, the queue. Without a reason like that, the structure sounds odd.
Be changes, the verb after going to does not
The structure has two parts, and only the first one moves. Be agrees with the subject, and after going to the verb is always the base form: no s, no ing.
I am going to call him.
She is going to travel.
They are going to cook.
One everyday detail. When the action itself is going somewhere, the doubling is usually avoided: people say I'm going to the cinema rather than I'm going to go to the cinema.
Negatives and questions
This works exactly as be does elsewhere: not attaches to am, is or are, and in a question that same form moves to the front. Neither do nor will is wanted here.
He isn't going to drive.
Are they going to come?
What are you going to do?
The mistakes everyone makes
The first is calling in do where be is already there.
Do you going to come? → Are you going to come?
The second is changing the verb after going to.
She's going to studies. → She's going to study.
The third is dropping the be at the front.
I going to call him. → I'm going to call him.
All three are caught by one look at the sentence. Only be changes, and everything after going to stays as it appears in the dictionary.
Try it right now
Name two things in English that you have already decided to do this week.
Then look around and say one sentence about something that is just about to happen, with the reason you can see right now.