Spontaneus decisions: Will is used for decisions at the moment of speaking.

I'm thristy, I will get some water.

Predictions: Will is used to make predictions without having evidenceso we use ell when we believe or think someting will happen.

I think it will be a great day.

Offers, promises and refusals: when we are dominated by some emotions, like happiness, anger or sadness, we use will to express an offer, a promise or a refusal.

I will marry you, I promise I will change.

We use going to for plans and intentions: going to is used when you have already decided to do something and will is at the moment of speaking.

I'm going to start a new book this weekend.

Predictions based on present evidence:

We use going to when we can see or know something now that helps to predicts the future:

Look at the sky it's going to rain.