'Will' and 'shall' are both modal verbs used to talk about the future or make suggestions about particular actions. Traditionally, 'shal'l is used with first person pronouns (I and we), while 'will' is used with second and third person pronouns (you, he, she, it and they).

Examples:
1. I shall be in Jos next weekend.
2. We shall provide all necessary assistance.
3. They will send delegates as requested.
4. Don’t mind them, you will check the document later.

The contraction of both 'will' and 'shall' is ‘ll, so when we use she’ll, we’ll, I’ll, etc., we don’t bother about the distinction because the shortened forms do not reflect which of the two is used.

The negative forms are 'will not' (won’t) and 'shall not '(shan’t: not common in American English).

Shall
'Shall' is used in suggestion, offer, request or to ask for advice in both British and American English.
Examples:
1. What shall I eat tonight?
2. Shall we approve the 2016 budget?
3. Shall I lead the protest?

'Shall' is also common in law but it’s losing its popularity, according to Garner’s Modern American Usage.
Example:
1. The plaintiff shall present the document and other evidence as and when due.

Will
'Will' is used when we order or ask someone to do something.
Examples:
1. Will you stop that silly prank!
2. Will you attend to them while I make tea?

To be polite in this context, use 'would' or 'could' in place of 'will'.

Changing the rule
When expressing strong determination to achieve something, 'will' is used with I and we, and 'shall' is used with you, he, she, it and they, which means there is a reversal of roles—another rule from traditional grammar. This rule is less followed in present-day English, but you should consider it in formal writing.
Examples:
1. I will not allow that to happen.
2. You shall attend to her when I say so!
3. They shall wait like everyone else.

Way forward
Generally, 'will' and 'shall' are now used interchangeably almost all the time. However, 'will' is preferred to shall in everyday American English, and in both Irish and Scottish English.