As long as you pay whatever tax is due by April 15, you will only be subject to an underpayment penalty for the later part of the year. That "penalty" is really just interest on the tax you should have paid. The current rate is 5% APR. Any tax unpaid after April 15 will be subject to interest and a late payment penalty of 0.5% of the unpaid tax for each month or part thereof that the tax remains unpaid. This is in addition to interest.
It is very easy to make a tax payment online. To do so, go to: https://www.irs.gov/payments
