If you pay your 2018 real estate tax in 2017, it is deductible on your 2017 federal return.
If you have not filed, to enter the information in TurboTax online edition, here are the steps:
1. After sign into your account, select Take me to my return
2. At the right upper corner, in the search box, type in real estate taxes then Enter
3. Select the 1st choice on the search results - Jump to real estate taxes
4. You will be directly taken to the screen, Did you pay property taxes in 2017?, answer Yes to enter information