going to
to express plans, intentions, projects...

We're going to get married as soon as possible.

What are you going to do now?

= We intend to get married

= What are your plans

predictions based on 'real evidence'
(that is what you can see, hear, smell,etc.)
Look at those black clouds: it's going to rain. According to what I see, it seems sure to happen
to express 'instant' decisions (phone starts ringing) - I'll answer! No premeditation, no plan (no time for it)
promises and offers

We will visit you next week.
I won't forget you.
Will you marry me? - Oh yes, I will...

will as 'want'
(promises, promises...)
predictions based on 'personal feelings'

fortune-teller: - you will be rich.
It will rain tomorrow.
I think he'll get the job.

no firm plan; that's what we think, imagine, expect, hope, bet, etc.



this is not exhaustive...