When I was in school, sometimes I used to steal/ take pens, pencils and many other things of my friends. Even I took money from my parents without asking them. How can I pay kaffara?

You have to pay for what you stole from people, or ask for forgiveness from them. This includes your parents. If you do not know the people from whom you stole, you have to pay an equivalent amount on their behalf to needy people.