Truthfulness as a personal trait is the steady habit of being honest in speech, in dealings and with ourselves. It includes not exaggerating, not hiding behind half-truths and being willing to say what is real even when it is uncomfortable. People who practise it are usually trusted, and they also tend to feel less burdened, since they have less to keep track of. The Gita counts it among the divine qualities.
Gita 16.2 lists harmlessness, truthfulness, calm without anger, freedom from fault-finding, compassion, gentleness and modesty as divine wealth, so respond to others with truth and kindness. Gita 17.15 describes disciplined speech as words that do not hurt and that are truthful, agreeable and helpful, and Gita 17.26 explains that Sat signifies what is real and good, so align your choices with truth and let integrity decide what you do.
The Gita also warns about what weakens truth. Gita 16.12 says unchecked desire corrupts the means we are willing to use, so examine how your desires influence the way you earn and spend. Gita 16.7 describes a mindset that has lost the ability to discern what should be done and what should be avoided, and that lacks purity, proper conduct and truth, so keep a clear moral compass by seeking guidance. Gita 18.42 lists honesty among qualities natural to contemplatives, and Gita 10.4 says qualities such as truth arise from the divine source.