“When the words that David spoke were heard, they repeated them before Saul, and he sent for him. David said to Saul. ‘Let no one’s heart fail because of him. Your servant will go and fight with this Philistine.’ Saul said to David. ‘You are not able to go against this Philistine to fight with him. You are just a boy. He has been a warrior from his youth.’”
Saul wanted to meet this guy who was talking so much so he sent for him. David said that he would volunteer to fight Goliath. Saul said no, because he was just a small boy and not a warrior like the Philistine. There is no indication here that they had met before or that David was his armor-bearer.
“But David said to Saul. ‘Your servant used to keep sheep for his father. Whenever a lion or a bear came and took a lamb from the flock, I went after it. I struck it down, rescuing the lamb from its mouth. If it turned against me, I would catch it by the jaw, strike it down, and kill it. Your servant has killed both lions and bears. This uncircumcised Philistine shall be like one of them, since he has defied the armies of the living God.’ David said. ‘Yahweh who saved me from the paw of the lion and from the paw of the bear, will save me from the hand of this Philistine.’ Saul said to David. ‘Go, and may Yahweh be with you!’”
David responded that as a shepherd, he had killed lions and bears with his own bare hands when they attacked his flock. This sounds a little like Samson. If Yahweh was with him, he would prevail. Saul finally said okay and prayed that Yahweh would be with him.
“Saul clothed David with his armor. He put a bronze helmet on his head. He clothed him with a coat of mail. David strapped Saul’s sword over his armor. He tried in vain to walk, but he was not used to them. Then David said to Saul. ‘I cannot go with these. I am not used to them.’ So David removed them.”
Saul wanted David to have his armor so he dressed him with the helmet, coat, and sword. So the armor bearer is now wearing the armor. However, David could not walk with all this stuff on him. So he took it all off.