Traditionally, 'shall' was used for I/we. Nowadys in British English, it is mostly used in questions in everday speech, though it is still used in formal language, so 'I shall' is fine in a business letter. American speakers don't use it much, or at all in many cases.
"Shall" can also be used with the second or third person (you, he, she, it, they) to indicate absolute certainty or (with "you") a strict order. For example:
"You shall go to the ball." (The Fairy Godmother to Cinderella: Cinderella believes it impossible that she might go to the ball, but the Fairy Godmother is certain that she will because she -- the Godmother -- will make sure.)
"You shall not pass." (Gandalf to the Balrog in Lord of the Rings. Gandalf is determined to stop the Balrog.)
The Biblical Ten Commandments are traditionally given in the form they appear in the old-fashioned King James Version. This uses a very old-fashioned singular form of the second person "thou", which is no longer used in everyday English (except in some British dialects). It looks like this:
"Thou shalt not kill."
This is a strict command, and the Israelites are expected to follow it. A more modern version would be:
"You shall not kill."
These days, however, this use of "shall" is old-fashioned and, when used for commands, far too direct and rude to be used in business correspondence.
Incidentally, many modern versions of the Bible -- mine included -- use this form for the Ten Commandments: "Do not kill".