In order to prevent incurring interest and penalties on the outstanding sum until it is paid off, you must pay the balance on line 37 of your tax return as soon as feasible.
The IRS provides two methods of online payment: Direct Pay and the Electronic Federal Tax Payment System (EFTPS). In addition, some retail partners accept payments made with cash, debit or credit cards, bank wire transfers, checks, money orders, electronic funds withdrawals, and even cash.
If you don't have the money to pay your tax debt in full right once, the IRS offers installment agreements that allow you to pay it off over time. A small fee and interest will be charged. But making payments gradually is far preferable to ignoring your debt and hoping it will go away.