You'll need to parse the string by separating the numbers and the operators (+,-,*,/, etc.). The numbers can be converted to integers or decimals using the .TryParse commands (e.g. Int32.TryParse()). The operators can be handled with a switch statement. The more complicated the types of expressions you want to be able to handle the harder it's going to be. You could use recursion to handle sub-expressions in parentheses.
Edit: I was going to find some sample articles but I see that Oded has already posted some.
