We use be going to when it is clear that something will happen. For example It is going to rain. He is going to fall. They are going to win.
We use will when we decide to do something at the moment of speaking. For example I will help you. I will do it. I will come.
In many situations, we can use both will and be going to. For example I will work tomorrow. I am going to work tomorrow. I will be there. I am going to be there.
We use be going to in about 3% of sentences in spoken English.
Read the book The Little Prince and practise grammar for Level 3.