What does this quote below mean? Atticus said something about the reasons why he was going to protect Tom Robinson?"The main one is, if I didn't, I...

First, this quote defines Atticus's character.  When he says that he could not hold his head up in town if he didn't defend Tom, he means that he has spent his life trying to do what is right and moral.  He knows, and he truly believes that his town knows, that the right thing to do is to defend and protect Tom from a racist, false accusation.

The second part of his quote--"I couldn't represent this county in the legislature"--establishes that Atticus does not simply try to live a personally moral life.  He wants his county to exemplify a group of people who are just and forward-thinking.  Because of his strong moral attributes, it would go against Atticus's ideology to know that his county really cares nothing about justice and to have to represent them with passion and loyalty to others.  Atticus sees Tom's case as a bellringer of how his county and small town will be remembered, and he wants no part in going down in history as one who contributed to prejudice and simple-mindedness.

Finally, Atticus is such an ideal father because he strives to teach by example.  All of the morals and lessons he imparts to his children are ones that he truly believes.  He doesn't simply tell them to live those morals because they will be better people; he lives those morals himself because he believes them himself and hopes to make the world a more just place.