To file your 2018 return in TurboTax Online, sign into your account and click "Take Me To My Return" and then select "File" on the left side of your screen. Walk through the filing steps and you will be given the opportunity to set up an auto debit payment for your balance due if you choose to e-file.
You also have the option to pay your balance due to the IRS with a check or money order or you can pay directly through the IRS Website here: https://www.irs.gov/payments.