This is historical information. Saul had been their king before David. (See: [[rc://en/ta/man/translate/writing-background]])
# You will shepherd my people Israel, and you will become a ruler over my people Israel
These two clauses mean basically the same thing and emphasize that Yahweh had chosen David to be king. (See: [[rc://en/ta/man/translate/figs-parallelism]])
Here ruling over the people is spoken of as shepherding them. Alternate translation: "'You will care for my people Israel" or "You will lead my people Israel" (See: [[rc://en/ta/man/translate/figs-metaphor]])