Sponteneous decisions: We used for desicions AT THE MOMENT of speaking

I am thirsty, I will get some water

Predictions: Is used to make predictions without having EVIDENCE, so we use will when we believe OR thing something will happen:

I think it will be great day!

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

I will marry you, I promise I will chance

We use going for plans and intentions: going to is used when you have already decided to do something

I am going to start a new book this weekend / They are going to visit their grandparents next month / She is going to study medicine

Predictions based on present evidence: We use going to when we can see or know something NOW that helps you to predict the future:

Look at the sky! It is going to rain (because you can see it) / He is driving to fast, he is going to crash / The team in playing pretty well, they are going to win the match

I am planning to do party in my house next month

The more carpet starts at 5:30pm

The english class starts at 11:30am

I am happy because the concert is tomorrow at 5:00pm

The uber arrives at 8:00pm