Future Simple Tense

English has several forms that can refer to the future. Three of there are "will, be going to, and the present continuous". The difference between them is not about near or distant future, or about certainty. The speakers choose a future form depending on when the decision was taken, and how they see the future event.
a. Form: S + Will/ Shall + Vinf
b. Uses:

• To express a future decision or intention made at the moment of speaking. Ex: I will give you my phone number. I have left the door open; I will go and shut it. I am too tired to walk now; I think I will get a taxi.
• To express an offer. Ex: I will carry your suitcase. Will you go with me? Will you open the door for me? We will do washing-up.
• It also expresses a future fact. The speaker thinks, "This action is sure to happen in the future". This use like a neutral future tense. The speaker is predicting the future, without expressing an intention, plan, or personal. Ex: tomorrow's weather will be warm and sunny. It will rain. He will finish his work next month. It will be right. Don't worry. If you go to Vietnam, you will see many interesting things there.

"Will": for a prediction can be based more on an opinion than a fact of evidence. It is often found with expression such as: "I think; I am sure...

c. "Tobe going to"
Form: S + Be Going To + Vinf
Uses:

• To express a future decision, intention, or plan made before the moment of speaking. Ex: how long are they going to stay in Vietnam? I am going to study harder for the test. I am going to write to Tom this evening. I know what you are going to say.
• When we can see or feel now hat something is certain to happen in the future, and based on present facts. Ex: look at these clouds! It is going to rain. Watch out! The box is going to fall. Carefully, the car is going to come. She is going to have a baby.
• The present continuous can be used in a similar way for a plan of arrangement, particularly with the verbs "go, come". Ex: she is coming on Friday. I am going home early tonight.

Note: The present continuous can be used to express a future

• Arrangement between people. It usually refers the near future. Ex: we are going out with Tom tonight. What are we having for lunch?
• Think of the things you might put in your diary to remind you of what you are doing over the next few days and weeks. These are the kinds of events that are often expressed by the present continuous for the future. The verbs express some kinds of activity of movement. Ex: I am meeting Peter tonight. I am seeing the doctor in the morning. We are going to party on Saturday night.
• Sometimes there is no difference between an agreed arrangement (present continuous) and an intension (going to). Ex: we are going to get / are getting married in the spring.