In depends on when in December you enrolled in Medicare.
If you were covered by an HDHP (High Deductible Health Plan) on December 1, 2017 and did not have any disqualifying coverage (such as Medicare), then you are considered to be under the HDHP for the entire month.
Assuming that you were under the HDHP for all 12 months in 2017, then your HSA contribution limit for 2017 has not been limited. This means that you would enter the information in the HSA interview (Federal->Deductions & Credits->Medical->HSA MSA Contributions) like you have in previous years.
If you were under Medicare coverage on December 1, 2017, however, you would not be qualified to contribute to the HSA that last month, and your annual HSA contribution limit would be reduced by 1/12th. TurboTax will handle this automatically.
If this is the case, in the HSA interview, you will come to a screen that asks "Did [name] have Medicare at any time in 2017?" (see screenshot below). Answer this question with "yes", then the next screen will ask you to list month by month what type of coverage you had - remember that it is the coverage on the first of the month that counts on the month.
One last note - when you enrolled in Medicare in December 2017, did the SSA backdate the enrollment 6 months? They often do, which causes no end of trouble for taxpayers.
