Will : 1 - Prediction ( Based on what you believe , think)
Tomorrow will be the best day of my life.
2 - On the spot decision
Oh! It's too hot in here . I'll open the windows.
3- Offers , promises , warnings , threats , requests
I will cook today , don't worry.
Going to : 1 - Predictions (Based on evidence in the present)
Look at those clouds! It's going to rain.
2 - Plan or intention
Today when I get home , I'm going to sleep
Present continuous for future use .
On Saturday I'm flying to Europe . I have already bought the tickets.
Shall I help you ?
Shall we go to the cinema today?