I used public transport without paying, how can I repent?

This is a human right. You have to get rid of this right before you go to the hereafter. Do like this: 1. Make sincere repentance for committing such a sin. 2. Try to find the owner of the vehicle. Ask him for halal, if he doesn't, give him what he wants. 3. If you can't find him, find his heirs. 4. If this is not possible, determine the amount that is haram in your pocket that day, even if it is approximately, and give that amount to a poor person as sadaqah. Insha'Allah you will not take those rights to the hereafter.