Being truthful comes from practice. To be truthful, we have to practice telling truth from childhood onwards. We need to be honest and sincere in what we say and what we do. If we make it a habit to say truth and never to lie, even in case of difficulty and conflict, then we remain truthful.
If we lie or become dishonest at any time, then we will need to accept our mistake, correct it and repair the damage. Then we come back on the path of being truthful. It requires a lot of courage and determination to be truthful as we encounter a number of conflicting situations. We may find that we benefit illegally if we lie or cheat at some times and no one knows. But the benefit from such an act will be less as compared to speaking truth over a long time. One may end up in a mess if one is not truthful to oneself and others in the society.