I. Prediction will

II. Decisions, intentions and arrangements.
1. Will.

Will is used to express a spontaneous decision or intention made at the moment of speaking.

e.g. I’ll phone you back in a minute.
2. Going to.

Going to is used to express a future plan, decision, or intention made before the moment of speaking.

e.g. When she grows up, she’s going to be a ballet dancer. We are going to get married in the spring.

NB. - In a formal style we use will rather than going to to talk about future events that have been previously arranged in some detail.

e.g. The meeting will begin at 9 a.m. Refreshments will be available from 8.30 onwards.
3. Present Continuous.

The Present Continuous is used to express personal arrangements and fixed plans (especially social and travel arrangements) when the time and place have been decided.

e.g. I’m having lunch with Brian tomorrow.

The Present Continuous is used to refer to arrangements between people. It is not used to refer to events that people can’t control.

e.g. It’s going to rain this afternoon (NOT It’s raining this afternoon).