I too learned it as Stella740pl did. Shall is used with the first person (I, we) and will is used with 2nd (you) and 3rd (he, she, it, they). I also learned that for emphasis, you change them.
Of course, I learned that many, many years ago and as with everything else, grammar has evolved. I think now, it is common to use will for everyday, informal use, and shall for more emphasis.