The $50 per month for hosting a foreign exchange student that you refer to is actually a tax "deduction," and not a tax "credit." (The difference here being that a credit is a dollar-for-dollar reduction in your actual income tax liability, whereas a deduction simply reduces the amount of your income subject to being taxed -- thus making a tax credit the much more valuable of the two.)
But, that aside, a detailed answer to your question is provided at the following clickable AnswerXchange link:
There you will find a significantly more detailed discussion of this particular deduction, as well as instructions on how to enter this information into TurboTax.
Thank you for asking about this.