First, I'd code up a function or two to handle the Caesar shift. If you don't have a clue about that, Wikipedia has a good write up on it. (and lots of other algorithms).
Get that working well, and then add in your functions for the reading of the input and make sure your output meets the requirements, as well.
Right now, we can't help you much, because you haven't studied the problem, and produced questions that show what has you stumped, at the moment.
When you have detailed questions, about specific code or logic that we can see, then we can be much more helpful.
