I will. No, I won't! I'm going to. No, actually, I'm not going to...
Do you really know the difference between the two future tenses? It's quite tricky (difficult) but not because of the grammar or any crazy irregulars, but because the correct future tense depends on the context of the situation! So, you really must know what exactly you want to say. The good news is though, that if you mix them up, it's not the end of the world grammatically, but someone might misunderstand what you are trying to say. Take a look at the chart below to help you understand the main differences between the two!
Will vs. Going to
Future plans (when everything is organized)
On Friday, she's going to visit her family.
Predictions: 80-100% probability (based on some current evidence)
Look at the sky! It's black! It's going to storm!
*going to is interchangeable with the present continuous when talking about future plans.
I'm visiting my family on Friday. (I know who, what time, where, etc.)
Future facts (things that always occur)
The sun will rise tomorrow.
Predictions: 50% probability (not based on current evidence)
I think in 2050 people will ride around on hoverboards.
I promise I will try harder.
I won't tell your secret, promise!
Requests (note that the order of the sentence changes in an interrogative phrase: will + subject + verb)
A: Will you help me clean the house?
B: Yes, I will / No, I won't, can't
I absolutely will not give you any more money until you pay me back!
Waiter: what can I get you?
Customer: I'll have a beer, please.
Shall I make you some tea?
(Do you want me to make you some tea?)
Formal or informal situations
Shall we order a pizza for dinner?
(Do you want to order a pizza for dinner?)
As you can see, will is used in many more situations that going to. As I said at the beginning of this post, if you confuse them, it's not a huge grammar mistake, but keep in mind that if you want to sound like a native speaker, remembering in which situations each are used will certainly make you sound more fluent! If you want to save this information, you can download my grammar reference below!