The law of karma is that you experience the consequences of your actions. Good actions have good consequences and bad actions have bad consequences. If you ignore slights and insults and refrain from harming anyone, and if you only do good actions you will only experiences good consequences from your actions. So far I have not said anything metaphysical, but now I will: After death, in your life review, you will experience how your actions affected other people from their perspective. You will feel all the pain and suffering you caused, and you will experience all the joy and happiness you caused. In a future incarnation, you might have to experience the harm you caused to someone in a previous life. When you harm another person, you harm yourself, but good actions lead to joy and happiness. So it is only self interest that should lead one to cultivate only good karma. That is the whole point of karma, it is our great teacher. From karma, one learns that it is more desirable to be good. That is one if the important lessons of spiritual development that makes us fit for the higher levels in the afterlife.
The main way of clearing up your karma, in addition to helping others, is forgiveness - ignore negativity instead of propagating it. Every action has consequences that spread from one person to another like ripples on a pond. If you absorb waves of negativity instead of propagating them, if you return good for evil, love for hate, if you break the (karmic) cycle of negativity, that action itself has consequences that spread out. If you find only being positive is hard to do, try meditation which should help you develop equanimity.