Since you ask in the Mathematics section, I assume you are looking for an optimization type of answer.
Mathematically speaking, you have to take an action that will be both positive and spread following a maximally growing function. These would be, for events involving people, usually exponential patterns.
You need to take actions that will both do good and set a positive example for the greatest possible number of people, so the amount of good actions taken by those exposed to the example will grow exponentially, reaching the greatest number of people in the smallest amount of time.