"Stand firm then, with the belt of truth buckled around your waist, with the breastplate of righteousness in place, and with your feet fitted with the readiness that comes from the gospel of peace." NIV translation
Paul begins to show us what our armor is and it starts with "the belt of truth". A belt is designed to keep your garments in place so that you do not trip over the ends of them. It is designed to be worn at the waist which is our balance point. The belt that we have to keep us from tripping and falling is the very Word of God. The second piece of armor is the breastplate which was designed to shield the heart and, as we see here, our heart is guarded by the fact that we have a righteousness that is granted by God and cannot be lost. Our feet are to be ready to carry the Word (gospel of peace) and to share it with others. When we understand what God has done for us (through a knowledge of His word), we will be ready to tell others about how they can have that peace as well.
The previous pieces of armor were all put on and then they just did their thing but the shield is different. The shield must be picked up and carried and it reminds us that this battle takes some effort on our part. A shield is not only used for defensive purposes but can also be used to attack when in close. As believers, our shield is our faith in Jesus for salvation and provision.
We know that we are God's chosen children and we know that He is faithful in caring for His children. Therefore, we know that "the evil one" can not overcome us.
"Take the helmet of salvation and the sword of the Spirit, which is the word of God." NIV translation
A helmet covers the head and protects the mind. The helmet of salvation speaks to the fact that we know that we are children of God and are saved. As we fight these spiritual battles, one of the favorite tactics of the devil is to try to get us to doubt our salvation but if we hold to our helmet we will not be affected by that tactic. We are also called to fight using the sword (word of God). Jesus gave us the example of this as He used the word of God to correct the devil when He was being tempted. (see Matthew 4)