All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Allaah Says (what means): {O you who have believed, fear Allaah and give up what remains [due to you] of interest, if you should be believers. And if you do not, then be informed of a war [against you] from Allaah and His Messenger. But if you repent, you may have your principal-[thus] you do no wrong, nor are you wronged.} [Quran 2: 278-279]
Therefore, dealing with interest is a major sin; anyone who gets involved in this sin wages a war against Allaah and His Prophet . The Prophet said: “Allaah has cursed the one who consumes Riba (i.e. usury or interest), the one who gives it to others, the one who records it and the one who witnesses it (its transaction).” He said: “All of them are equal in sin.” [Muslim]
However, if you signed the papers without knowing that it is an interest-based loan, Allaah will forgive him. The Muslim scholars agreed that anyone who commits a sin unknowingly is excused. The evidence for this ruling is the Hadeeth that a person committed sin beyond measure and when he was going to die, he left a will: burn my dead body and then cast the ashes to the wind and to the ocean believing that Allaah will not be able to resurrect him. No doubt, he did not know the Might of Allaah covers everything. Despite of the ignorance of this man, Allaah pardoned him. [Muslim]
Thus, you are not permitted to pay the interest if you can avoid it. Otherwise, you may pay it if you are compelled to do so, since Allaah forgives the one who is forced to commit what is forbidden. The Prophet said: “Allaah has forgiven my nation for things they forget, or do by mistake, or which they are forced to do.” [Ibn Maajah & Al-Bayhaqi]
You have to inform your father of the bad consequences of dealing with Riba.
On the other hand, we commend your feelings of sorrow and concern for committing this sin. In fact, this is a sign of a good and faithful heart.
We ask Allaah to grant you and all Muslims steadfastness.
Allaah Knows best.