Albert Barnes Commentary Psalms 101:1

"I will sing of lovingkindness and justice: Unto thee, O Jehovah, will I sing praises." — Psalms 101:1 (ASV)

I will sing of mercy and judgment - That is, in the psalm which he was about to compose, he would make these the burden of his song; he would, in fact, by stating his views as to the regulation of his own conduct, commend these virtues—mercy and justice—to mankind, and celebrate their value. He who himself adopts the principles of mercy, kindness, truth, and justice, as his own guide, commends these virtues to mankind in the best way possible. No language can do it effectively, unless one practices these virtues oneself.

Unto you, O Lord, will I sing - As commending and approving these things; as having put it into my heart to practice them; as displaying them in your own higher administration: for a father of a family, or a magistrate, is only the representative of God.