In fact, the difference between the two forms is not so well-defined; if you use either one, it will be understood that you are describing an action in the future. The problem is that English has lost an inflected verb form to communicate the future time, so helper words must be used. "Will" is used to indicate the future time, as is "shall." They are both a little bit more formal than the "I am going to <verb>" construction, with "shall" indicating a little bit more certainty or resolve than "will."

As far as time scale and certainty are concerned, at least as much information about how far in the future your action will be performed, or about how certain that action is, will be communicated by non-verbal means (facial expressions, hand gestures, and so on), and by the way you "sing" the sentence (prosody). So I could say, for instance "I'm going to have a wild romance with Juliette Binoche," but anyone hearing me say that would know that that event is certainly not on my calendar for the next week!

By the way, I'm interested in your 50/50 Italian/English exchange. I studied Italian intensively just for a few weeks about 10 years ago to prepare for a trip, but haven't used the language since. I do know a lot of French and Spanish, though, so I have the "hooks" to hang Italian on. If you're still looking for partners, please contact me. I'm a very precise English speaker, having received writing awards from two universities, and having gotten a perfect score on the Graduate Record Examination. My mother was an English teacher at one of the New York City "genius" high schools. In my childhood, we very commonly ate our dinner with a dictionary (or two or three) on the table, there to support our arguments about word meanings and derivations. 

when you want to say you want to do something in future, you should use "will".

And when you have a specified schedule for doing something in future, you should use "I'm going to".

for example: "I'll call my friend to invite him for my birthday party."

or: "I'm going to call my friend to invite him for my birthday party."

In the first sentence, you just show you have decision to call your friend.

But in the second sentence, you show you have a special plan to call your friend, for example, you want to call him at a certain time like tonight or today afternoon or tomorrow morning for any reason you have.

Thanks a lot for your answers, both. Now is more clear about future.