It depends upon the type of donation, but more than likely your contributions should be able to be carried over to next year. Therefore, you should enter into TurboTax the charitable contributions you made in 2017. If they are not able to be deducted this year, TurboTax will determine if they are allowed to be carried over to your next year's return. And, if they are allowed next year, TurboTax will transfer them over for you when you transfer your data to complete next year's return in early 2019.
Per IRS Publication 526:
You can carry over any contributions you can't deduct in the current year because they exceed your adjusted-gross-income limits. Except for qualified conservation contributions, you may be able to deduct the excess in each of the next 5 years until it is used up, but not beyond that time. Your total charitable deduction for the year to which you carry your contributions generally is limited to 50% of your adjusted gross income for that year
https://www.irs.gov/pub/irs-pdf/p526.pdf
