Will is normally used after pronouns such as I, we, and you. Will indicates your intention to do something, such as I will take out the trash, or a firm fact of a consequence of something; you will get into trouble if you do that.
The word shall is mostly used in a question. Although it can be used as an aid to a refusal, such as “You Shall Not Pass!” it’s mostly used as the beginning of a question, such as Shall I take out the trash? or Shall we go?
An easy way to decide which to use is by determining how formal you want to be. Will sounds more demanding, but shall is the more formal manner of speaking.
Thanks to Grrly Girl for suggesting this post!