If you mistakenly overpaid your 2018 taxes that over payment will be refunded after you file.
Unfortunately, you will still owe tax for 2017.
For online payments,
IRS.gov > Make a Payment > Direct Pay > Make a Payment >
Reason: Tax Return or Notice
Apply Payment To: 1040, 1040A, 1040EZ
Tax Period for Payment: 2017.
