"Then Saul gave David his own armor—a bronze helmet and a coat of mail. David put it on, strapped the sword over it, and took a step or two to see what it was like, for he had never worn such things before. “I can’t go in these,” he protested to Saul. “I’m not used to them.” So David took them off again. He picked up five smooth stones from a stream and put them into his shepherd’s bag. Then, armed only with his shepherd’s staff and sling, he started across the valley to fight the Philistine." I Samuel 17:38-40
Once you figure why you are fighting the giant. Then you need to know how to fight him. What weapons will you use?
That is the first decision David had to make once he decided the giant must fall. King Saul had an idea for him. Even though he didn't want to fight the giant, he was willing to donate his armor and his weapons.
When your time comes to face the giant you will find other people have ideas how you should do it and weapons they are willing to loan. They don't want to face the giant but sure are free with their advice!
David was wise enough to know that if he was going to bring down the giant he couldn't do it with other people's armor. He could not face the giant with someone else's sword. In a big fight you need a familiar weapon. The shepherd boy had become an expert marksman with a sling and a stone. Because he had practiced and prepared and become proficient with this weapon, God was able to augment his skills to bring down a giant. God can't use what we haven't prepared. David understood that it was really a spiritual battle and that the battle was the Lord's. He understood what St. Paul knew:
"A final word: Be strong in the Lord and in his mighty power. Put on all of God’s armor so that you will be able to stand firm against all strategies of the devil. For we are not fighting against flesh-and-blood enemies, but against evil rulers and authorities of the unseen world, against mighty powers in this dark world, and against evil spirits in the heavenly places. Therefore, put on every piece of God’s armor so you will be able to resist the enemy in the time of evil. Then after the battle you will still be standing firm. Stand your ground, putting on the belt of truth and the body armor of God’s righteousness. For shoes, put on the peace that comes from the Good News so that you will be fully prepared.In add ition to all of these, hold up the shield of faith to stop the fiery arrows of the devil. Put on salvation as your helmet, and take the sword of the Spirit, which is the word of God." Ephesians 6:10-17
God's battles must be fought with His weapons. You can bring down physical giants with spiritual weapons - David did. And you can bring down spiritual giants with spiritual weapons but you can't bring down spiritual giants with physical weapons.
Killing giants takes the right reaons and the right weapons. David knew. Now you do, too.