Thou hast delivered me from the strivings of the people; and thou hast made me the head of the heathen: a people whom I have not known shall serve me. The strivings of the people - Disaffections and insurrections among my own subjects, as in the revolt of Absalom, the civil war of Abner in favor of Ish-bosheth, etc.
The head of the heathen - ראש גוים rosh goyim, "the chief," or "governor, of the nations;" all the circumjacent heathen people; all these were subdued by David, and brought under tribute.
A people whom I have not known - The people whom he knew were those of the twelve tribes; those whom he did not know were the Syrians, Philistines, Idumeans, etc. All these served him, that is, paid him tribute.