Is this for a class assignment? If so, I'd use whatever you find the most manageable to code. You will have to parse the text for the operators and maintain precedence. Other than that pretty straight forward. Start one step at a time.
Can you read in a line of text, that would be a good place to start.
Can you parse the string, seperating the numerics from the operators (even if that parsing means nothing at all, can you at least look at each character and tell if xxx is a 3 digit number or y is an operator?)