The bottom line: use will for everything, and you'll be safe from the grammar police.
If, however, you want to emphasize your intention, use shall. A famous example of this is in John Kennedy's inaugural address:
Let every nation know, whether it wishes us well or ill, that we shall pay any price, bear any burden ...There are some other rules -- if you happen to live upstairs at Downton Abbey or plan to meet the queen.
Here's how they say it:
In British English, there has been a traditional rule that shall is to be used when the subject is in the first person (I or we), and will in other cases. In practice this rule is commonly not adhered to by any group of English speakers, and many speakers do not differentiate between will and shall when expressing futurity, with the use of will being much more common and less formal than shall. In many specific contexts, however, a distinction still continues.So it's a rule, but not a rule. Got it? Fowler allows as how the rule "is so complicated that those who are not to the manner born can hardly acquire it."
Winston Churchill surely knew the rule. I don't know if he was honoring it when he promised:
"We shall go on to the end. We shall fight in France, we shall fight on the seas and oceans, we shall fight with growing confidence and growing strength in the air, we shall defend our island, whatever the cost may be. We shall fight on the beaches, we shall fight on the landing grounds, we shall fight in the fields and in the streets, we shall fight in the hills; we shall never surrender ... "His use of shall with the first person we is proper. An American, however, hears an emphasis. I suspect he did as well. As it should be.