We use 'be+going+to+base form of verb' to express any action in future that has already been decided. For example: I have decited to visit the Zoo on Sunday next. I'll say," I'm going to visit the zoo on Sunday."
There are different ways to express the future, one of these is be going to. Will indicates an intention or a future plan to do sthg, on the other hand be going to expresses a firm decision to do sthg.
'Be going to' is used for plans and decisions in the near future. Shall/will are used for sponteaneous responses and promises in the far future. Present Continuous is used for arrangements in the immediate future.
What are you going to do this coming weekend? I'm going to visit my mum.