Here's some simple advice: Forget about using 'shall' in statements. You never actually need to say 'I shall'. If you do, it may be incorrect, or simply unnatural. If you use 'I will', or more commonly 'I'll', you'll be fine. The only time we commonly use 'shall'' is in questions when you are offering or suggesting. 'Shall I carry your bag for you?' 'Shall we have lunch?'. Apart from these situations, you really don't need to worry about 'shall'.

I shall be in Japan, but you will be in New York The verb will is used in a number of ways, but we chiefly use it (followed by the infinitive of another verb) to talk about the future: 1. Hopefully, we will have a very good election. 2. Life in the village will never be the same again. 3. When will you go to New York? 4. I will be in Japan this time next year. The negative form is will not, which is often shortened to won’t: 5. I won’t be away for long. 6. These phones will not be available till next year. 7. We will not understand the situation until the facts have been collected. The die-hard grammatical sticklers amongst you might have already started jumping up and down after reading the above examples. Why? Well, in traditional British grammar, the rule is that will should only be used with second and third person pronouns (you; he, she, it, they). With first person pronouns (I and we), the ‘correct’ verb to talk about the future is shall. This means that strictly speaking, examples 1, 4, 5, and 7 are ungrammatical, and should instead read: Hopefully, we shall have a very good election. I shall be in Japan this time next year. I shan’t be away for long. We shall not understand the situation until the facts have been collected. In practice, however, and especially when speaking, people are more likely to shorten will and shall when these verbs are used with pronouns (we’ll, they’ll, etc) and therefore there’s no need to worry too much about the distinction when referring to the future, unless you’re writing in a very formal situation or having to conform to an organization’s style guide. Equally, not all varieties of British English use ‘shall’ in these senses. Some varieties of English, including Scottish and Irish English, tend to use ‘will’ instead of ‘shall’ when talking about the future, no matter if it’s with the first, second, or third person pronoun.

In American English, for all intents and purposes, they mean nearly the same thing, but "shall" is not commonly used. Shall is more common in British English and can sound pretentious. 'I will go to the store tomorrow" "I will one day own my own house" "We shall return very soon" "I shall become rather fat if I keep eating"