How did you select to pay it? By check or direct debit from you account? Check your return. If you were suppose to send a check it should have printed out a 1040 V payment voucher. The sooner you pay it the interest will stop accruing.
You can pay the IRS directly at
https://www.irs.gov/payments
Or print a blank 1040V voucher
https://www.irs.gov/pub/irs-pdf/f1040v.pdf
