Last year I paid $500 to the IRs in advance of filing my taxes this year. How do I indicate that amount on Turbo Tax to decrease the amount I owe now?

You made an Estimate payment?

You can type estimates paid in the search box at the top of your return and click Find and it will give you a link to Jump To it.

OR
To enter Federal or State Estimated Taxes Paid, including a state estimated payment made in  Jan 2018 for 2017 (or Jan 2019 for 2018), go to
Federal or Personal (for Home & Business version)
Deductions and Credits
Then scroll way down to Estimates and Other Taxes Paid
Estimated Taxes - click the Start or Update button

Estimated payments go on 1040 Schedule 5 line 66
And show up on 1040 line 17

You'll enter the estimated tax payments you made by typing "estimated tax payments" into the search box and clicking on the "Jump to" link in the search results. (see sample screenshot below)