Well, the word "Shall" is meant as the word "Will". So if I say "I shall buy you some bread while I'm at the market", you could replace the word shall with "will". The word "Should" is meant as an obligatory term. When you say "should", you are saying that you are telling yourself or are being told by others to do something. So in this case, you could say "I should buy you some bread while I'm at the market". There's a difference when you say the word "should" versus "shall".
I hope I helped! And if you need more information, just tell me! :D
