The basis of your Roth IRA account is the amount you have contributed to the account.
You should only pay tax and the 10% penalty for early withdrawal on the earnings of the account.
After you have entered your form 1099-R, TurboTax will take you back to the summary page titled Your 1099-R Entries.
On that page, click Continue and follow the TurboTax until you arrive at the page titled Enter Prior Year Roth IRA Contributions where you can enter the amount of your past contributions.