Theory:

There are three ways of talking about the future in English, WILL, GOING TO and THE PRESENT CONTINUOUS.
For example:
I think I WILL eat fish tomorrow.
I am GOING TO eat fish tomorrow.
I am EATING fish with my sister tomorrow.

Exactly which form is used, and when, and why, can sometimes be confusing.

The choice of which form is used is related to how sure you are, and it is important to remember that WILL is used with the FIRST CONDITIONAL.
For example:
If you study you will pass your exams. (Which is not a definite situation).

WILL
So, let's start by looking at how to use WILL.
WILL is followed by the infinitive of the verb without TO, like I WILL CALL YOU.
It is used for situations where you are not 100% sure, when you say I THINK.
For example:
I THINK he WILL call. (You don't know for sure.)

This example is also a prediction, but without any evidence.
So WILL is used if there is an element of doubt. (For a guess.)

You can also decide to do something without planning it, and WILL is used for that, but in the contracted form.
For example:
Wait, I'LL help you.

GOING TO
GOING TO is used when you are sure, or have some evidence to justify what you are saying, like when you have already made plans.
For example:
I am GOING TO speak to my boss. (You have made a definite decision.)
I know David is GOING TO be late. (He called to tell me).

PRESENT CONTINUOUS
Finally, using the present continuous for the future.
I AM SPEAKING ENGLISH NOW is the present continuous.
It can also be used for the future, for plans that you have made and written in your diary.
It is very similar to GOING TO, but is not used for guesses, so it is different from WILL.
For example:
I AM SEEING my dentist tomorrow. (I am sure because it is in my diary).