You use
'will' when...
1. ... you offer to do sth. or spontaneously decide to do sth.
(I will help you move next week.)
2. ... you promise something.
(I will buy you that game if you pass your test.)
3. ... you predict that something will happen.
(It'll be dark this time tomorrow.)
Shall you only use when it's a question starting with I en we.