Java IO help!!!!!

I have three assignments for class: 1. use a stack with exceptions to solve the problem of balanced parentheses, brackets, and braces; 2. use the same definition to solve the problem of evaluating a postfix arithmetic expression; 3. use the same definition to solve the problem of converting an infix arithmetic expression to its equivalent in postfix form.

There are some basics my teacher seemed to have left out. He explained the BufferedReader etc for input, but I cannot figure out how to take a line of input from the keyboard and push each individual character onto a stack. Is there a simple way of doing this that I have missed? It's difficult to go much further with my programs without knowing this.

Thanks for any and all help!!!!