I had a job last year which I no longer have and used majority of the money to pay for my kaffarah for missed fasts. I also used some of the money to buy gifts for my family and some on myself for vacation.

Unfortunately, I completely forgot about Khums until recently. How do I calculate how much khums I have to pay? I no longer work and even when I was working my main living expenses were paid for by my father, so it was not necessary for me to to have a job at the time

You only have to pay khoms if by the end of your fiscal year you had extra money.