Going to is not a tense. It is a special expression to talk about the future.
Structure of Going to
The structure is:
subject + be + going + infinitive
The verb be is conjugated (past, present or future).
Use of Going to
Going to - intention
We use going to when we have the intention to do something before we speak. We have already made a decision before speaking. Look at these examples:
In these examples, we had an intention or plan before speaking. The decision was made before speaking.
Going to - prediction
We often use going to to make a prediction about the future. Our prediction is based on present evidence. We are saying what we think will happen. Here are some examples:
In these examples, the present situation (black sky, the time, damaged car) gives us a good idea of what is going to happen.