David was the eighth and youngest son of Jesse, from Bethlehem. He was a strong, healthy lad, apparently with an uncommon characteristic among his people of the time - red hair. He was a Shepherd from an early age, and developed his courage and fighting skills by defending the flocks from the wild animals, including lions and bears, that preyed upon them. The free time that being a shepherd provided also allowed him to develop two other skills, that of music and poetry. David was a warrior, and a writer of psalms.
The incident with Goliath the Philistine is probably one of the most told "Bible stories" of the Old Testament. When the Israelites were at war with the Philistines, the two armies faced each other from opposite hills with the Valley of Elah between them. Every morning for forty days, the mighty Goliath (he may have stood over 9 feet tall) challenged the Israelites for someone to come out and fight him, but none would go out. One day, David, who was actually then too young for the army, arrived with some deliveries for his older brothers. He heard Goliath and immediately volunteered to fight him.
When David explained to King Saul that he had been fighting fierce animals all his life, he convinced the king that he could defeat the Philistine. Perhaps by then the king was willing to try anything to get out of the embarrassing situation, and even if David were not successful the Israelites could always belittle his lack of success with the excuse that David was "just a kid."
After turning down an offer of the king's own armor, which was too big for him, David went down to the creek and got five suitable stones (five, not just one - apparently he was not absolutely sure of himself) for his sling. The rest of the story is famous. He killed Goliath with a single perfectly-accurate shot, perhaps with a little help from an angel - the stone didn't just rebound off the giant man's thick skull as would naturally be expected, but actually penetrated with the power of a modern high-velocity bullet. Upon seeing their hero defeated, the Philistine army made a disorderly retreat (i.e. they just dropped everything and ran), giving the Israelites then in hot pursuit the victory. The entire story can be found in 1 Samuel 17:1-58.
The success made David very popular in Israel, and when Saul's erratic and unwise character ruined him as Israel's ruler, The Lord had David appointed king. David's monarchy was of course planned and decided long before as part of the human ancestry of Jesus Christ.
Despite all of the many difficulties and shortcomings that David experienced, politically and even in his own family during his reign, David's rule over Israel was a time a great advancement for the nation. Jerusalem was taken from the Canaanites and made the capital, and a vast amount of territory was gained in all directions. He established Israel as a force to be reckoned with in that part of the world. Ultimately, he set the stage for his son Solomon to take Israel to the highest point in its history (see Solomon's Kingdom).
Fact Finder: How many years was David king?
(a) 7 (b) 10 (c) 25 (d) 40
1 Kings 2:10-11