Instead of this:
You may want to try
charGrade = Console.ReadLine();
here is the problem I for see you having with your code. A char represents one character but, what happens if a user types in AB? Your code is going to throw an exception =\ I would suggest using a string unless you are going to write in error checking/handling so that if more then one char is passed in, you pass only one char to your variable.
I will only tell you how to do it, not do it for you.
Unless, of course, you want to hire me to do work for you.
Read this if you want to know how to get a correct reply for your question:
^^Took that from planoie's profile^^
^^Modified text taken from gbianchi profile^^