"Be going to" is used like an auxiliary verb. We use "be going to" just like other auxiliary verbs (e.g. "can" or "should"):

1. "I can go";
2. "I should go";
3. "I am going to go". 

When we use auxiliary verbs, we change the meaning of the sentence a little. In the examples above, using different words changes the sentence from being about ability (1), to obligation (2) and then to a plan (3):

1. "I am able to go";
2. "I had better go";
3. "I plan to go".

Future using the present

There are some different ways to talk about the future in English. Here are a few:

1. "I will do";
2. "I am doing";
3. "I am going to do"; etc. 

When you use "will", it is the simplest way to talk about the future. But sentences two and three use present language. In English, we sometimes use present language to talk about future plans. It is to create a feeling that, even though the action is in the future, it is already happening. It is real to us now. It means that we can 'see' the action coming. It means that it is planned, decided or somehow happening now. This language creates a feeling that something has already started. If you look at "be going to", you can see that actually it is a present tense pattern (like number two above). If I say "I am watching TV", I am talking about what I am doing right now. When you say "I am going", it means you are on your way. This is a very important idea.

When you say something like, "I am going to eat steak for dinner," you are saying that you have already made this plan. You already decided what you want to eat. Somehow, you have begun to prepare for your meal—maybe you bought some meat from the supermarket or you chose a restaurant to visit. Even just thinking about it is the base for your action later. It is a very small start, but you can see that you will be eating steak tonight.

How to use "be going to"

It is easy to make a sentence with "be going to". Just follow this pattern using a subject, be, going to and a verb:

When to use "be going to"

"Be going to" is informal, and usually used in casual speech and writing. There are four main times you can use "be going to":

1. plans;
2. predictions (things you can see will happen);
3. commands; and
4. refusals.

1. Plans

The most common way to use "be going to" is to talk about plans. It says that you have decided already.

1. "I am going to quit my job."
2. "She is going to live in India."
3. "Steve is going to come on Sunday."
4. "They are going to buy a new house."
5. "Are you going to go out tonight?"
6. "What are they going to do this weekend?"

In all of these cases, the choice or plan was made before. Because the plan has been made, it is like it is already happening.

2. Predictions

When you look at something and can guess what will happen, you can use "be going to". Here are some examples:

1. You look at the sky, and see there are dark clouds: "It is going to rain."
2. Your friend is standing on an office chair (one with wheels), trying to change the light. It looks very dangerous: "You are going to fall!" 
3. Watching your favourite team play, they are many points ahead: "We are going to win!"
4. You are nervous about a test: "I am not going to pass..."

You can see these things coming. You can see the future. Using "be going to" is a way to show that something now tells you what will happen in the future:

1. There are dark clouds, so you know it will rain.
2. The chair is not stable, so you know it is dangerous.
3. There is not enough time for the other team to get enough points, so you know your team will win. 
4. You did not study hard enough, so you know you will fail.

3. Commands

Another way to use "be going to" is to tell someone what to do. Again, you are so sure that it will happen, you use present language. There is no choice for the other person. It's the plan, and no-one can change it:

1. "Mark is going to finish that report today."
2. "You are going to be here at 9 a.m. tomorrow."
3. "They are going to listen to what I have to say, whether they want to or not!"

When you read these sentences, often you put the STRESS on "going to":

1. "Mark is GOING TO finish that report today."
2. "You are GOING TO be here at 9 a.m. tomorrow."
3. "They are GOING TO listen to what I have to say, whether they want to or not!"

This is a very forceful way to speak. It can sound aggressive or angry. If you want to be more polite, you would say something different:

1. I would like Mark to finish that report as soon as he can.
2. Would you mind being here at 9 a.m. tomorrow?
3. Please listen to what I have to say. 

4. Refusals

Because "be going to" sounds very strong, you can use it when you have a strong idea about what you want to do:

1. "I am going to leave, and you can't stop me!"
2. "I am going to tell them what you did!"

More often, it is used to refuse or say what you will not do:

1. "I am not going to work overtime again this week!"
2. "I am not going to lend you any more money!"