There are three main future tenses in English, "will", "going to" and present continuous.
Will future is used for two things, prediction and spontaneous promises.
I will never walk on the moon (prediction)
I'll ring you tomorrow (spontaneous promise)
Going to future is used for predictions, especially based on immediate evidence, and plans.
He's going to fall! (prediction, based on immediate evidence)
I'm going to buy a new car next year. (plan)
Since your question was particularly about when not to use will, let me give a couple of instances. We don't say things like
? I'll buy a new car next year.
when we talk about plans, so that is one case where "will" is not used. Also if I saw someone tottering on a roof I probably would not say
? He'll fall
we normally use "going to" when there is some kind of immediate evidence.
By the way, the other future, present continuous used for the future, is used for appointments and other fixed plans:
I'm having lunch with Megan Fox tomorrow.
There are a lot more details but as I said in the comments you'd better look in a grammar book, because this site isn't exactly chock-a-block with ESL teaching experts. For complete details I strongly recommend "English Grammar in Use" by Raymond Murphy.