After you file

I imagine the liability is not extensive for a 19 year old. I just want to let you know 2 things:

1.The IRS has a quick  How Do I File a Deceased Person interactive assistant. It is very detailed and may give you additional information. 

2. An executor can make a request for discharge from personal liability for a decedent's income, gift, and estate taxes. The request must be made after the returns for those taxes are filed. To make the request, file Form 5495. For this purpose, an executor is an executor or administrator that is appointed, qualified, and acting within the United States.

Within 9 months after receipt of the request, the IRS will notify the executor of the amount of taxes due. If this amount is paid, the executor will be discharged from personal liability for any future deficiencies. If the IRS hasn't notified the executor, he or she will be discharged from personal liability at the end of the 9-month period.

NOTE: Even if the executor is discharged from personal liability, the IRS will still be able to assess tax deficiencies against the executor to the extent he or she still has any of the decedent's property.

For additional assistance, please refer to Publication 559, Survivors, Executors, and Administrators

I am very sorry for your loss.