You will need to file an amended return, form 1040X and any new schedules regarding your investments. The IRS will bill you for the late payment penalty and interest.
The late payment penalty is one-half of one percent per month based on the amount not paid. Current interest is 4%.
You may request an abatement of the penalty if you have a reasonable excuse for the mistake.
Filing an amended return can be a bit tricky so if you have doubts about doing it yourself, a CPA or Enrolled agent may be money well spent. It really depends on the amount of tax you think you may owe. A professional will also be experienced in requesting the abatement of the penalty.
Thank you for your question. Please feel free to reply if you need additional information.
Your tax bracket depends on your filing status, exemptions for dependents, and deductions. If I assume you are single and take the standard deduction, then most of your additional income will be taxed at 15% (16,000 * .15) and the rest at 25%, for a total of $3,400. Check to see if there is any foreign taxes withheld that you can take as a credit against this tax.
The sooner you file, the lower your interest and penalties will be. H & R Block does have some offices open now. You can enter your zip code on their website to find one.
If you want to wait until your local office opens, they are usually not too busy early in January.