Am I obligated to pay Zakat on my money, and how do I calculate it for past years, knowing that I was unaware of the Nisab for silver, and my money was kept by my father who spent it on family expenses, making it a debt, and he has other money that, by itself, does not reach the Nisab for silver?

Firstly: It is not obligatory to pay Zakat for past years, based on the preponderant opinion that paper currency's nisab (minimum taxable amount) is estimated by the lower of the two nisabs (gold or silver), in consideration of the poor's welfare. Therefore, whoever did not pay it previously based on the gold nisab owes nothing.

Secondly: Zakat is obligatory on the minor's wealth if it reaches the nisab and a hawl (lunar year) passes over it. If your father paid Zakat on your behalf, then you owe nothing.

Thirdly: The money your father took with the intention of repayment is considered a debt owed by him, and Zakat on debt is applicable to it. If your father is solvent and acknowledges the debt, then Zakat is obligatory every year and is paid when the money is available. If he is insolvent, then you pay Zakat for one year upon receiving it.

Fourthly: For Zakat to be obligatory, the money must reach the nisab either by itself or by being combined with other money, gold, silver, or trade goods. If the money with your father reaches the nisab, then any money you own on which a hawl passes becomes subject to Zakat, even if it is less than the nisab on its own, because it reaches the nisab by virtue of being combined with other assets.