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:
1. Jo has won the lottery. He says he's going to buy a Porsche.
2. We're not going to paint our bedroom tomorrow.
3. When are you going to go on holiday?
In these examples, we had an intention or plan before speaking. The decision was made before speaking.