I had an extra $100 per month taken out of my check last year. How do I get that money back?

You mean you had an extra $100/month withheld for taxes?  That would have been included in the amount you saw in box 2 of your W-2, so your refund was calculated based on the amount you had withheld.  There is no place on a W-2 to list "extra" money you had withheld;; it is just added to the amount your employer withheld for taxes.

You do not necessarily get that back. You only get the excess (if any) of your total withholdings over your tax liability.