When do you use which one:
am/are/is + going to + verb
We are going to invite your aunt to the wedding
am/are/is + verb - ing
I'm playing tennis with Joy tomorrow
will + infinitive
I think I will ask Luke for help
We use this one for fixed future arrangements, which have already been planned and prepared
We use this one to talk about future intentions or plans, which may still be changed
We use this one for spontaneous decicions made at the moment of speaking, often in reaction to a new situation