Did IRS send you a letter?
If so, pay that amount.
If not, pay the amount shown on your form 1040.
Look at your 2018 Form 1040. Send in/pay the amount that is shown on the tax return on line 22. Since it is late, you will also owe interest and penalties, but the IRS will bill you for that later, based on how much you owed and when they receive your return and payment.
Did you file the 2018 return--but not pay the tax due? Or do you still need to print and file the return too? It cannot be e-filed; it must be mailed if you are filing it now. Do you know how to access your 2018 return?
Many people have multiple TT accounts and forget how to access them. Log out of the account you are in now.
Account recovery
https://myturbotax.intuit.com/account-recovery/
https://ttlc.intuit.com/questions/1901535-forgot-your-turbotax-online-user-id-or-password
Or did you use the desktop version of TurboTax? If so, the files are on your own hard drive or any backup device you used like a flash drive.
https://ttlc.intuit.com/questions/1901486-how-many-turbotax-accounts-do-i-have
https://ttlc.intuit.com/questions/1901535
https://ttlc.intuit.com/questions/1901659-find-your-tax-data-file-in-mac
https://ttlc.intuit.com/questions/1900721-find-your-tax-data-file-tax-file-in-windows
To get a copy of your previously filed returns prepared with online TurboTax https://ttlc.intuit.com/questions/1900748-how-do-i-get-a-copy-of-a-return-i-filed-in-turbotax-online
f you have federal tax due you can pay by mailing your payment with the 1040V voucher, having the payment taken out of a designated bank account, or you can pay directly on the IRS website.
https://www.irs.gov/payments/direct-pay
https://ttlc.intuit.com/community/tax-payments/help/how-can-i-pay-my-federal-taxes/00/26212
TT is not updated for late payment penalties and interest.
check out this IRS website.