The foundation of character training is honesty. Without honesty, every other virtue falls flat. One becomes known as a liar and no amount of kindness, purity, or respectfulness can overcome that. Because lying can become a habit and as a child grows older becomes harder and harder for a parent to be certain about, it is important to teach and enforce the importance of speaking the truth at all times.
This must be addressed as soon as your child is able to answer questions.
The first step is to teach the difference between true and false. You want to be certain your child understands what is the truth and what is a lie. This isn’t as difficult as child psychologists want us to believe. Simply spend a few sessions over a few days asking simple questions and answering. Here is an example:
At lunch, you down with a peanut butter and jelly sandwich. “Is this a peanut butter and jelly sandwich? “Yes, it is. That is the truth.” Then also say, “But what if I said, “No, it isn’t. It’s a ham sandwich.” That would be a lie! How terrible! We need to always tell the truth! You can always trust me to tell the truth, just like I can always trust you to tell the truth.”
You should find many ways to give examples over several days. During that time, make an effort to not ask questions of your child that gives him the opportunity to lie, especially questions like, “Did you hit your sister?” (When he knows that punishment will come with an affirmative answer.) Once you are certain he understands the difference between truth and falsehood you can dramatically --- enforce “Truth at all times.” Punish for misbehavior as needed but make it clear that lying is a much greater offence. If you take a few days or weeks to consistently watch and then question your child about everything, you can establish a habit of truthfulness that will pay off when he is older and you cannot watch every moment.