The concepts of will and going to future can be very confusing.
They both refer to events in the future and are often interchangable with no difference in meaning.
There are some rules as to when use which form.
Let's first look at how both tenses are created:
will future: will/will not (won't) + verb
Example: He will swim. He will not/won't swim
going to future: be (am/is/are) + (not) going to + verb
Example: He is going to swim. He is not/isn't going to swim.
Now, let's see when to use will and when to use going to future:
Prior Plan - The decision was made before:
Evidence/Signs - It's very likely to happen:
Very near future - Something is about to happen:
WILL or GOING TO
Finally, put your new knowledge to use! :-)
Try not looking at the table until you are done!