How is truth presented in the face of all the lies in the book, Monster?

Most of the important witnesses against the defendants have something to gain by lying under oath. Does this mean that they are actually lying? The witnesses for the defendants also have something to gain, in that their relative and/or friend will not be taken from them. Does this mean that they are actually lying? How can anyone tell, other than through the witnesses' behaviors on the stand? Does Steve tell the truth about his own innocence? Why did O'Brien feel the need to coach Steve about his way of answering questions? Is Steve simply napve or is he really guilty? Perhaps directly to the point, should a person be too truthful in a trial that might put this person into prison for over twenty years, or is it ethical to avoid the prison time? After all, Steve did not kill anybody. He barely participated in the robbery, if at all. He might have been a foolish young man, trying to be tough like King, but is the punishment too harsh? After twenty years in jail, Steve would likely be a hardened criminal, if he does not kill himself first. What is the ethical thing to do in Steve's situation, tell the absolute truth or tell a lie to get out of trouble? Perhaps he was not in the drugstore the day of the murder, but Steve suggests that he was.