By relying upon the true nature that resides within each person and is good (i.e., the imago Dei), a person can continually practice self-reflection, self-cultivation, and moral responsibility. After enough practice, that person's life approximates an ideal whereby s/he has actualized her or his full potential and is manifesting a state of righteousness.
It is easier said than done: go where peole are, talk to them, live with them, but also try to read as much as possible about the field and experiences you are having. Gain the trust of people, and understand them from "within". Do not dispise anyone, and certainly not a priori, and try to learn as much as you can from others lives and experiences...
When you ask about being "useful" and "contributing," I think you are speaking of effectiveness. In my experience, individual action has little effect on the world, although there may be rare exceptions. For the most part organizations and institutions shape the social environment. Even the charismatic individuals who have most influenced history (people such as Karl Marx or Gandhi) have only done so through the organizations that were created to promote their ideas. Thus, working to create or change organizations is the only way I know to make a difference in the world.