To your first contribution. For your contribution basis TurboTax wants to know the sum of all your regular Roth IRA contributions prior to 2018 minus all of your Roth IRA distributions prior to 2018.
If you are reporting a Form 1099-R that has code T and your first Roth IRA contribution was for some year prior to 2014 or you are reporting a code Q Form 1099-R, TurboTax doesn't need to know your contribution basis to determine the taxable amount of your distribution.
