Theory:

THE FUTURE

There are THREE ways of saying something in THE FUTURE:
Using WILL
For example:
I think it WILL rain.

Using GOING TO
For example:
It is GOING TO rain because the sky is very dark.

Using the PRESENT CONTINUOUS
For example:
I can't see you tomorrow because I AM SEEING Maria.

Which form is used depends on HOW SURE you are of the situation.

If you are not sure, you might say I THINK.
If you have some evidence to help you decide, then you have some reason to be sure.
If you have made some plans in your diary, the you have made a decision and are quite sure.
If you decide to do something without planning it, then it is not the same as a pre-planned definite event.

WILL is used for situations where you are not sure, where you have not planned anything, and where you have no evidence to justify what you are saying or when you make a decision to do something at that moment.
For example:
I THINK it WILL rain.(If you think, then you are guessing.)
I WILL call you tomorrow. (This depends on many things, it is not definite, it is MAYBE.)

This sentence comes from the first conditional:
I WILL call you IF I have time, or IF I can, or IF I remember.

When you use will for decisions made at that moment, it is used in the CONTRACTED FORM 'll.
For example:
Wait, I'LL call a taxi. (Because you didn't plan this action, WILL is used, and in the contracted form)

GOING TO is used if you are sure, or if you have made a definite decision, or have some reason to justify your opinion.
For example:
It is GOING TO rain because the sky is full of dark clouds. (Your reason is the colour of the clouds.)
I am going to the dentist next week.(Not MAYBE because you have made an appointment and YOU ARE SURE you are going.)

The PRESENT CONTINUOUS is only used for something YOU are going to do, and have PLANNED to do.
You cannot say:
It is raining later because the sky is dark.
You have to say:
It is GOING TO RAIN because the sky is dark.

You have planned to have lunch with maria tomorrow so you can say:
I AM HAVING lunch with Maria tomorrow.