Not sure about the online version, but in the desktop version just start a new return and go through the interview, it will ask if you are filing for a deceased taxpayer. You need to be a person authorized to file the return, either appointed by a court, or if there is no proceeding be named in his will/trust etc. Also, see IRS form 1310 and instructions.
Please see this IRS article, How to file a final tax return for someone who has passed away.
On the final tax return, the surviving spouse or representative will note that the person has died. The IRS doesn't need any other notification of the death.
Usually, the representative filing the final tax return is named in the person's will or appointed by a court. Sometimes when there isn't a surviving spouse or appointed representative, a personal representative will file the final return.
Representatives who aren't court-appointed must include Form 1310, Statement of Person Claiming Refund Due a Deceased Taxpayer to claim any refund. Surviving spouses and court-appointed representatives don't need to complete this form.
See also this TurboTax article. TurboTax, both Desktop and Online versions, provides an entry in the Personal Info section to indicate that the person is deceased. The final return is filed on the same form that would have been used if the taxpayer were still alive, but "Deceased" is written at the top of the return followed by the person's name and the date of death. The deadline to file a final return is the tax filing deadline of the year following the taxpayer's death.
I am very sorry for your loss.