The leader of the Israelite army, Saul overheard what David had said and went to talk to him. David convinced Saul to let him fight Goliath but Saul had dressed David in his own armor before letting him fight.
Go and let the lord be with you
"David strapped his sword over the tunic and tried to walk, but he was not used to all this armor. “I cannot walk in these,” David said to Saul. “I am not used to them.” So David took them off. " (1 Samuel 17:39) And so David went off to fight Goliath with his sling and five smooth stones
Goliath saw David and he despised him and moved to attack him. David ran to meet him, selected a stone, slung it and it flew into Goliath's head killing him. Having seen their Giant die, the Philistine army ran away leaving the Israelite army victorious thanks to the guidance of the Lord