According to the author of the book, David was very brave and had just defeated the giant Philistine, Goliath. Abner, a helper of King Saul, took David to see the king, and David was holding the giant's head to show what he had done.
1 Samuel 17:57: And as David returned from the slaughter of the Philistine, Abner took him, and brought him before Saul with the head of the Philistine in his hand.
These verses say David stood before Saul and became his armor-bearer, indicating a pre-existing relationship, whereas 1 Samuel 17:57 suggests Saul did not know David.
1 Samuel 16:21-22: And David came to Saul, and stood before him: and he loved him greatly; and he became his armourbearer.