How do I know when to use “will” and “going to” to make a sentence in the future?

Question by John A: How do I know when to use “will” and “going to” to make a sentence in the future?
It always seems as though people use them interchangeably, at least in speech, while some say that
a good writer had better use them well.

Some advice would be much appreciated.

Best answer:

Answer by Fraser
I will be going to the shops later.
I am going to the shops later.

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