WILL
Simple Future has two different forms in English: "will" and "be going to." Although the two forms can sometimes be used interchangeably, they often express two very different meanings. These different meanings might seem too abstract at first, but with time and practice, the differences will become clear. Both "will" and "be going to" refer to a specific time in the future.

Examples:

You will help him later.
Will you help him later?
You will not help him later. lear. Both "will" and "be going to" refer to a specific time in the future.
GOING TO
Use
1) planned actions in the future
We are going to sing at the party.

2) You are certain that sth. is going to happen in the future.
Look at this car! It is going to crash into the yellow one.

Examples

Affirmative sentences:
I am going to play handball.I'm going to play handball.
You are going to play handball.You're going to play handball.

Negative sentences:
I am not going to play handball.I'm not going to play handball.
You are not going to play handball.You're not going to play handball.You aren't going to play handball.

Questions:
Am I going to play handball?
Are you going to play handball?
credits:English Grammar Book