Phronesis

It is not true that in order to do good, we must know the good thing to do. We can, after all, do good by accident. Perhaps, for example, in driving down the street we accidently hit and kill a rabid dog that otherwise would have gone on to bite people. Or perhaps our neighbor has performed several favors for us, such as cutting our lawn when we had a broken leg, and we wanted his good deeds to be acknowledged so we nominated him for a good neighbor award or the like, which involved publication of his picture, and it turned out that someone recognized him as a serial rapist that had terrorized a nearby community, and subsequently he was arrested, prosecuted, and jailed for years,  saving many woman from a horrible assault. Even more extreme, we might do good when our intent was to do evil. Perhaps I wanted to harm a neighbor and I poisoned his drinking water well, with the intent of great harm, but as it turns out, the poison I poured into his well was the precise mixture to counteract poison someone else had just poured into the well, and the water was safe to drink. 
These examples reveal that doing good need not always involve virtue on our part. Virtue requires knowing the right thing to do in a given circumstance, and then doing that thing. Now it is certainly true that in endeavoring to do good we in fact cause harm, but so long as our intent was honorable, and so long as we had undertaken prior to acting all that would be expected of a reasonable person in respect to understanding the situation and what the proper course of action should be, then most often we are held blameless if our act does cause unintended harm.
Essential for our acting virtuously is what the ancient Greeks understood as phronesis, which can be defined as practical wisdom. Aristotle placed particular importance on this trait, for without wisdom on what is the right thing to be done in a given situation, we can hardly put our other virtues to good use. Aristotle understood phronesis as a skill we develop through practice, through reflecting on our acts and seeking to thereby gain wisdom on how we might improve on our future actions. Hence, if we possess phronesis, then we know how to act - appropriately - in a given situation. The act itself is identified as praxis, which signifies the actual process of performing what phronesis informs us is the right thing to do in the particular situation. 
To improve, therefore, our ability to act appropriately in the world, we must do all we can to gain in practical wisdom, we need, that is, to maximize our phronesis.