Let's treat David the way the text itself treats him — not as a stained-glass king, but as the most fully documented human being in the entire Old Testament, sins and psalms both left on the page without editing. We'll walk through his life in order. Where the New Testament picks something up — and almost nothing in David's story goes unclaimed later — that gets pointed to as we go. Where a voice worth hearing has something to add, it's brought in by name.
Here's the shape of the whole life first. David is likely in his mid-teens when Samuel anoints him in Bethlehem, a shepherd, the youngest of eight sons. He kills Goliath not long after. He spends roughly a decade as a fugitive from Saul before becoming king over Judah at thirty, then over all Israel a few years later. He reigns forty years total — seven and a half over Judah alone, the rest over the united kingdom — and dies "in a good old age, full of days, riches, and honour" (1 Chronicles 29:28), handing the throne to Solomon. Hold that whole arc loosely as we go, because the man at the beginning and the man at the end are recognizably the same person, and that continuity — not moral perfection — is most of what makes him worth studying closely.
Samuel is sent to Jesse's house in Bethlehem under cover of a sacrifice, because Saul is still alive and would kill any rival outright. Jesse parades seven sons in front of the prophet, and each time Samuel is sure he's found the one, God stops him: "Look not on his countenance, or on the height of his stature... for the LORD seeth not as man seeth; for man looketh on the outward appearance, but the LORD looketh on the heart" (1 Samuel 16:7). Notice that this line isn't abstract theology dropped in for atmosphere — it's the narrator explaining, in real time, why the youngest, least likely son in the house is about to be anointed while his father hasn't even bothered calling him in from the sheep. David is anointed privately, in front of his brothers, and the text tells you plainly what happens next: "the Spirit of the LORD came upon David from that day forward" (1 Samuel 16:13). There's no coronation, no throne, nothing visible changes. Just a shepherd boy sent back out to the flock with an anointing nobody outside that room knows about, and years still to wait before any of it becomes public.