David makes it clear before the battle, before he strikes Saul and cuts off his head, that the weapons before them weren't going to win the battle.
The Phillistines don't honor the agreement, they flee, and the Israelites are going to have to fight them anyway. And they litter the road with the Phillistines. David's sling didn't kill all of those Phillistines. God did it. The victory over Goliath is just a prelude of what's about to happen next.
What are some times that God helped you to conquer a giant?
What are some times God brought a victory in your life that other people saw and attributed to God?
This isn't the last giant David will face: Saul and his own son each try and take him down. But God helps him conquer each "giant."
David once sung a song about his beasts, circling about his feet. Jesus quotes the same song on the cross, as he slays the biggest giant of all, death and separation from God.
You have the same weapon David had, it is not YOU who are ill equipped for the fight, it's the enemy...who showed up with a knife to a God fight!