1 Samuel 17:32-51:

"Don't worry about the Philistine," David told Saul.  "I'll go fight him!"  "Don't be ridiculous!" Saul replied.  "There's no way you can fight this Philistine and possibly win!  You're only a boy, and he's been a man of war since his youth."
But David persisted.  "I have been taking care of my father's sheep and goats," he said.  "When a lion or a bear comes to steal a lamb from the flock, I go after it with a club and rescue the lamb from its mouth.  If the animal turns on me, I catch it by the jaw and club it to death.  I have done this to both lions and bears, and I'll do it to this pagan Philistine, too, for he has defied the armies of the living God! The LORD who rescued me from the claws of the lion and the bear will rescue me from this Philistine!"
Saul finally consented.  "All right, go ahead," he said.  "And may the LORD be with you!"
Then Saul gave David his own armor - a bronze helmet and a coat of mail.  David put it on, strapped the sword over it, and took a step or two to see what it was like, for he had never worn such things before.
"I can't go in these," he protested to Saul.  "I'm not used to them." So David took them off again.  He picked up five smooth stones from a stream and put them into his shepherd's bag.  Then, armed only with his shepherd's staff and sling, he started across the valley to fight the Philistine.
Goliath walked out toward David with his shield bearer ahead of him, sneering in contempt at this ruddy-faced boy.  "Am I a dog," he roared at David, "that you come at me with a stick?" And he cursed David by the names of his gods.  "Come over here, and I'll give your flesh to the birds and wild animals!" Goliath yelled.
David replied to the Philistine, "You come to me with sword, spear, and javelin, but I come to you in the name of the LORD of Heaven's Armies - the God of the armies of Israel, whom you have defied.  Today the LORD will conquer you, and I will kill you and cut off your head.  And then I will give the dead bodies of your men to the birds and wild animals, and the whole world will know that there is a God in Israel! And everyone assembled here will know that the LORD rescues his people, but not with sword and spear.  This is the LORD'S battle, and he will give you to us!"
As Goliath moved closer to attack, David quickly ran out to meet him.  Reaching into his shepherd's bag and taking out a stone, he hurled it with his sling and hit the Philistine in the forehead.  The stone sank in, and Goliath stumbled and fell face down on the ground.
So David triumphed over the Philistine with only a sling and a stone, for he had no sword.  Then David ran over and pulled Goliath's sword from its sheath.  David used it to kill him and cut of his head.

When the Philistines saw that their champion was dead, they turned and ran.  Then the men of Israel and Judah gave a great shout of triumph and rushed after the Philistines, chasing them as far as Gath and the gates of Ekron.  The bodies of the dead and wounded Philistines were strewn all along the road from Shaaraim, as far as Gath and Ekron.  Then the Israelite army returned and plundered the deserted Philistine camp.  (David took the Philistine's head to Jerusalem, but he stored the man's armor in his own tent.)
As Saul watched David go out to fight the Philistine, he asked Abner, the commander of his army, "Abner, whose son is this young man?'
"I really don't know," Abner declared.
"Well, find out who he is!" the king told him.
As soon as David returned from killing Goliath, Abner brought him to Saul with the Philistine's head still in his hand.  Tell me about your father, young man," Saul said.
And David replied, "His name is Jesse, and we live in Bethlehem."