Both "going to" and "will" are used to talk about future actions but they are not exactly the same.
Let's see the difference :
"Will" is used when we decide to do something at the time of speaking. We have not decided before. Let's see the following example:
Ninda : My laptop is blinking. Can you fix it for me?
Prana : Certainly, but I'm busy at the moment. I'll fix it tomorrow.
"Going to" is used when we have already decided to do something. We have decided before the time of speaking.
Let's see the following example :
Wisnu : Can you fix Ninda's laptop? It's blinking.
Prana : Yes, I know. She told me so. I'm going to fix it tomorrow.
Will or Going To ?
1. Ann : I am thirsty.
Jill : Wait here. I ... get some water for you.
2. Adi : I've decided to change the color of this wall.
Sue : Oh, have you? What color ....(you/paint) it?
3. Dan : Are you going to the supermarket ?
Amanda : Yes, I .... (buy) something for dinner.
4. Prana : What would you like to drink ?
Edy : I... ( have ) coffee, please.
5. Ben : I don't know how to use this cellphone.
Denny : It's easy. I ...show you.