David sprang to this new prominence by his single-handed defeat of the Philistine champion, the giant Goliath. 
It so happened that that the Philistines gathered their armies against the Israelites led by their king, Saul.
The Israelites and Philistines each stood on a mountain facing each other with a valley between the two armies.

And there came out from the Philistine camp, a champion named Goliath of Gath, whose height was six cubits and a span. He had a helmet of bronze and was armed with a heavy coat of mail around his bust and legs. He had a great spear and his shield beard walked before him.

Goliath challenged the Israelite army to produce a champion who would fight with him in single combat. ‘If he wins we will be your slaves; but if I win and kill him, you will be our slaves… I dare you to pick someone to fight me.’ David meanwhile, a shepherd boy rather than a soldier, had been sent by his father to the front line to bring rations to his elder brothers who fought in Saul’s army. He heard the giant’s boast, and offered to be the one to fight him while everyone else was afraid and fled, much to the ridicule of this own brothers who scoffed at him.

When Saul was told of the boy’s suggestion, he sent for him. And David said to Saul “Let no man’s heart fail because of him. Your servant will go and fight this Philistine.” But Saul discouraged him saying he was but a youth before Goliath who was a man of war from his youth. But David said to Saul, “Your servant used to keep sheep for his father. And when there came a lion, or a bear, and took a lamb from the flock, I went after it and struck it and delivered it out of his mouth. And his he arose against me, I caught him by his beard and struck him and killed him. I shall kill Goliath like the same lions and bears and the LORD will deliver me from the hand of this uncircumcised Philistine, just like He delivered me from the paws of the beasts”.

Saul was not convinced, yet for want of anyone better and persuaded by the boy’s faith and courage, King Saul agreed. ‘Go,’ he said to him, ‘and the Lord be with you.’

Then David took his staff in his hand and chose five smooth stones from the brook, and put them in his shepherd’s bag, in his wallet; his sling was in his hand and he drew near to the Philistine. 
And the Philistine came on and drew near to David, with his shield-bearer in front of him. And when the Philistine looked, and saw David, he disdained him; for he was but a youth ruddy and comely in appearance.

And the Philistine said to David, ‘Am I a dog, that you come to me with sticks?’ And the Philistine cursed David by his gods. The Philistine said to David, ‘Come to me, and I will give your flesh to the beasts of the field.’  Then David said to Philistine, ‘You come to me with a sword and with a spear and with a javelin; but I come to you in the name of the Lord of hosts, the God of the armies of Israel, whom you have defied. This day the Lord will deliver you into my hand, and I will strike you down, and cut off your head; and I will give the dead bodies of the host of the Philistines this day to the birds of the air and to the wild beasts of the earth; that all the earth may know that there is a God in Israel, and that all this assembly may know that the Lord saves not with the sword and spear; for the battle is the Lord’s and he will give you into our hand.’

When the Philistine arose and came and drew near to meet David, David ran quickly toward the battle line to meet the Philistine, And David put his hand in his bag and took out a stone, and slung it, and struck the Philistine on his forehead; the stone sunk into his forehead, and he fell on his face to the ground.

So David prevailed over the Philistine with a sling and with a stone, and struck the Philistine with a slink and with a stone, and struck the Philistine and, and killed him; there was no sword n the hand of David. Then David ran and stood over the Philistine, and took his sword and drew it out of its sheath, and killed him, and cut off his head with it. When the Philistines saw that their champion was dead, they fled.

David achieved immense popularity with the people of Israel. Saul fired by jealousy and fighting for his throne, pursued him relentlessly. Finally when David was thirty, Saul was killed in battle and David’s kingship was at last established.