David beats Goliath in a classic case of unconventional warfare.
So, the Philistines are camped on one hill and the Israelites on another, with the Valley of Elah in between them. Goliath, who is ten feet tall, leaves the Philistine camp. He's wearing very expensive armour. He starts shouting at the Israelites: Why are ye come out to set your battle in array? am not I a Philistine, and ye servants to Saul? choose you a man for you, and let him come down to me./ If he be able to fight with me, and to kill me, then will we be your servants: but if I prevail against him, and kill him, then shall ye be our servants, and serve us.
/ And the Philistine said, I defy the armies of Israel this day; give me a man, that we may fight together. (v. 8-10).
The Israelites are afraid and refuse to accept the challenge. David's three oldest brothers are in Saul's army, but David is back in Bethlehem tending his sheep. The taunting goes on for 40 days, until Jesse sends David with a care package for his brothers. He happens to arrive in the middle of a battle, which ends when Goliath approaches and the Israelites flee.
David rather arrogantly asks who is this uncircumcised Philistine, that he should defy the armies of the living God? and is filled in on the backstory. The others don't take kindly to this interloper, and news eventually gets back to Saul. Saul calls for him and is unimpressed by what he sees: a kid with a rock, but David insists he's killed lions with his bare hands protecting his flock and besides he's got god on his side. So Saul armour suits him up, but David finds the gear cumbersome and goes out with his staff, some stones and a slingshot.
Goliath laughs when he sees the puny boy the Israelites have sent, then charges him. He's surprised when David, rather than turning on his heel, also charges, then hits him in the head with a slingshot, killing him. Then he kills him again with his own sword. Yeah, I'm thinking a couple of legends got rammed together here. He cuts off Goliath's head for good measure. The Philistines, seeing their champion fall, run off. The Israelites give chase.
Saul asks who David's father is, but suddenly no one knows. David himself finally arrives and explains.