Going to...
'Going to' is used to express future events which are very likely but not certain.
Let's look at these verbs before we start.
We looked at how to make the future simple:
I will go to the station.
She will eat pasta.
This form is used when we are certain the event will happen. If there is a doubt that you will do what you say we use the form:
Subject + verb 'be' + going to + present tense verb
This also very natural because there's always some doubt.
Future simple vs. 'going to'

Pizza
Subject + will + present form verb
Subject + verb 'be' + going to + present form verb
I will eat pizza.
I am going to eat pizza.
This is certain. 100% it will happen.
This is not certain but very likely.
Note: 'going to' is not a modal verb but it is easier to think of it as one
Going to - Negative
Subject + will + not + present form verb
Subject + verb 'be' + not + going to + present form verb
I will not eat pizza.
I am not going to eat pizza.
This is certain. 100%.
This is not certain but very likely.
100%
80%
I will eat pizza.
I am going to eat pizza.
20%
I am not going to eat pizza.
0%
I will not eat pizza.
Advanced explanation
'Going to' expresses an intention to do something. You have a plan or desire to do something. For example, you might say 'I'm going to the supermarket' but that is not certain. It could rain and you may choose not to go. A negative example is 'I'm not going to drink alcohol'. Here it is your plan that you won't drink, but you might. You might decide to have a glass of wine. Whereas, if you say 'I will not drink alcohol', it is certain you won't.
Going to - Examples
I'm going to eat fish and chips.
I'm going to visit the Harry Potter studio.