The simple future has 2 different forms in English :
1. Will
2. Be going to

Will
Form
Will + Verb

Will expresses a :
1. Voluntary action ⇒ I will help him to finish his homework.
2. A promise ⇒ Don’t worry, I won’t tell anyone.
3. Threat ⇒ If you do not finish the project, I will need to fire you.
4. Refusal ⇒ She won’t do anything I say.

Be going to
Form
am/is/are + going to + verb

Be going to expresses a  :
1. Prior plan ⇒ I’m going to see my girlfriend this weekend (it as already decided)
2. Signs of somethings ⇒ I am feeling sick, I think I’m going to throw up.

Be going to : plans
It expresses an idea of what someone wants to do in the future (realistic or not).

CAREFUL! :
1. For predictions, both will and am going to can be used.
2. The present continuous can also be used for something that we have already decided.
e.g.
The band is visiting the UK next May
I am taking the bus to London tomorrow.

RECOMMENDATION : Check B1U4L2 to learn in more detail about using « will » in the future.