Your taking money from your father’s briefcase without his knowledge was a reprehensible action and an evil deed. Praise be to Allaah Who has guided you and enabled you to repent. Part of your repentance is to put back the money that you took.
If telling him about it will make him angry or make him not trust you, then put the money back by any means without telling him, such as putting it in his briefcase and so on.
The scholars of the Standing Committee were asked: Since I was young, if I saw my father putting something away, whether it was money or things that were useful, I would take it, and my father did not know about that. After I grew up, I began to fear Allaah and I stopped doing all that. Now is it permissible for me to tell my father about that or not?
You have to return the money and other things that you took from your father, unless it was something simple like pocket money, in which case it does not matter. End quote.
Fataawa al-Lajnah al-Daa’imah, 15/352
They were also asked: (24/355): If a person steals some money then he wants to repent, and return the money to its owner without him knowing it, what is the ruling on his repentance?
If the situation is as you describe and he is sincere in his repentance and regrets what he did, and he has resolved not to do it again, then his repentance is valid and his repentance is not affected if the one from whom it was stolen does not know who returned his money. End quote.
And Allaah knows best.