bantu donggg, terimakasihhh

1. be + going to
a. I am going to have a 1 on 1 match with Jay after school
b. I already bought the ticket for the movie, so I am going to watch a movie tonight
c. My stomach hurts a lot, I think I am going to throw up
d. It's 78% vs. 22%. That candidate is going to win the election
e. It's going to rain tomorrow

2. will
a. I don't know that the clothes looks good. I'll try those
b. I will tell your mom that you are smoking
c. I promise I will behave next time
d. I will probably watch a TV tonight
e. The forecaster says it will rain tomorrow

3. (Pardon me if I'm wrong, okay? )
Differences:
'be + going to' is normally used to indicate the future but it still have the connection with the present. Meaning, you already made a plan before (see point a and b) or you know the result based on the evidence or experience (see point c and d)
However, 'will' is used where there is no connection with the present. For point a, 'will' is used when you have sudden decision (you didn't plan it before, you just decide it at that moment when you look at the clothes). For point b (threat) and c (promise), it's likely 'I' will do those, but don't know when. There's no plan, no time limit.

Similarity:
Both can be used to predict the future (see point e in both examples and point d in example for 'will')

Hope that help. Good luck!