I assume you have no other Traditional IRAs. If you do, your distribution is not tax free.
After entering your 1099-R (a little later, not immediately), you will be asked if you ever made any non-deductible (after tax) contributions to your IRAs. That's where you will enter you "basis". Note that you will also be asked for you year end balance in ALL your traditional IRAs.
The calculation of the taxable amount (the earnings) will show on form 8606.
